So, to expand this a bit from just a quick response last night when I caught this thread.... a lot of us have some similar thoughts, opinions and experiences. I think this topic is fascinating because I get to approach it from two angles - one, as a school orchestra director who mostly works with elementary aged students and their families, of which a big part of the onboarding process is getting a hold of an instrument; and two, as a double bassist who's been playing long enough to least have a somewhat discerning ear and is wondering where the sweet spot is for the amount of bass that I want to make me happy. So:
What are the things that determine a bass is a “student” bass? I suppose a 1/4 or 1/2 size might be a big indicator? But what else? Cheaper hardware? Finger board material?
What are the drawbacks of student basses? Advantages?
Again, remembering that "student" is a marketing term - I call bulls*** on it. At the lower range of price points (heck, at any price point) there are quality instruments and crappy instruments. A quality student (and by student, let's say up to a few thousand dollars) instrument is typically going to be made with high quality materials and be constructed by or at least under the supervision of someone who has some semblance of what it takes for an instrument to sound decent and hold up over the long term to the rigors of regular use. Anything not meeting this standard, IMO, is not something I would recommend or want to play myself. Quality materials, at a bare minimum, include good wood for the body (even if that just means the proper type of plywood for cellos and basses) and neck, and an ebony fingerboard. A quality instrument should also have at least a halfway decent setup that doesn't require taking it to a shop right away to get it play relatively easily.
For a comparison, the first "student double bass" popping up on my Amazon search is a Cecilio something-or-another- oak fingerboard, an immediate no-go for me because I know there's a high likelihood they're cutting significant corners elsewhere if they won't even put an ebony board on it. I've seen too many of these instruments as a school orchestra teacher. I don't like them (but I tolerate them, because it's the best that some of my families can afford, I recognize that, and I try to make those instruments sound and play that best I can). Eastman, on the other hand, which is a brand I know well and trust because they are consistently well made, includes an ebony fingerboard on even its lowest entry model, the VB80.
Now, before I get flak for ragging on Cecilio, Mendini, or whatever other Amazon CCIs/ISOs are out there - obviously it's a free capitalist market, you can spend your money how you want to, and if you're satisfied with such an instrument, play it in good health and flame away until I am extra crispy. Let me state again, however, that I work as a school orchestra director and have done so for over 10 years. My data set for comparison is the roughly one thousand or so orchestral string instruments that have passed through my program in that time. The difference in quality between a rental instrument such as Eastman, Strobel, Yamaha, Scherl & Roth, or many of the house brands sold exclusively through string speciality shops vs. these Amazon instruments is quite obvious. Ask me which ones are far, far more likely to break just by looking at them, even from something as simple as the glue at the neck joint failing after a few months because it isn't strong enough. Ask me which ones come out of the box with strings so high off the fingerboard you could drive a truck underneath them, if they even come with the bridge cut and slotted at all. Ask me which ones literally feel spongy at the tuners or need to be retuned multiple times in the same 30 minute lesson because the wood is so soft, green, flimsy, or just not properly braced. It's ridiculous.
One caveat - I will say that some of these Amazon brands do offer instruments that approach the quality level of Eastman and similar. They are also similar in price though (in this price tier of instruments, I do believe that you generally get what you pay for) and I've only seen maybe a handful of them. In my community at least, families who choose to buy outright typically aren't shelling out that much money for their kid's first instrument.
The string instrument business is also notoriously opaque at all levels. There's not nearly as much brand recognition as there is for many other products. Eastman is perhaps the best known brand among student instruments IMO, and even then I imagine that many have not heard of them. Again, as I mentioned before, string specialty shops have a foothold in many communities still, and many of them work directly with manufacturers or at least middlemen who only sell through these speciality shops to offer a "house brand" for school rentals, entry level purchases, etc. to capture the slice of the market that isn't looking for a professional-level instrument. With it being something of a niche market, unless you have experience and know what to look for, it can be very difficult to know whether an instrument is high quality or not. This is one reason why it drives me up a wall that there's a plethora of social media influencers peddling these ISOs - getting paid by these manufacturers to do it, I have no doubt whatsoever - and trying to compare them favorably to instruments that are actually well made.
So tl;dr - the advice we've been hocking to newbies on here is still the most sage - figure out your price point and go shopping with someone who has experience and who you trust.
I think we are discussing a very clear distinction between a "student double bass" that someone would buy who intends (or hopes) to go to an elite music school with the intent to move into a high-level professional double bass career, and a "student double bass" that someone who is informally learning to play double bass (with or without a local teacher) has bought with the intent to play with local bands.
The common "student double bass" at your local music store -- if you can find such a local music store -- would probably not be the higher level "student double bass" even though it is labeled the same as one. I think we're talking about a build quality and value difference of one or more thousand dollars.
We've got a terminology problem in part because many if not most retail outlets -- even some of the most respected ones -- tend to call whatever lowest level they can afford to carry a "student double bass" or something very closely implying the same thing.
Maybe there's a better way to identify these two distinctly different "student double bass" levels of quality here so it won't be quite so confusing???
Yes, again - context is so important to this discussion. Most of the schools around where I live are stocked, if they are fortunate, mostly with laminate basses from a high quality brand like Eastman or similar. The rare hybrid or carved bass might be reserved for a first-chair player.
My general experience in my geographical area resonates with yours in that string instrument and double basses in general don't get much attention at all at the local omnibus music store, and it probably skews the general layperson's perception of what constitutes student-level instrument quality. A trip to a string speciality shop, or ideally a double bass shop, is much more likely to result in finding good instruments.
IMO, in the orchestral world, laminate basses are probably enough for the casual K-12 student who just wants to play in their school orchestra and has no aspirations of post-secondary study. Anyone getting serious with the bow, however, needs to be looking at a carved top at minimum.
Point of comparison, though - as @Wasnex mentioned above, the same old Kays that were yesterday's high school orchestra bass are some of the most vaunted instruments in use in the bluegrass/folk/Americana world. The same can be said of some of those old Juzek and other early 20th century central & eastern European instruments - were they not the cheap mass-produced basses of their day as the European economy was trying to dig itself out of the ravages of WWI? Now some of the better examples of that era rank among the most favored jazz basses out there and command a price suggestive of this level of desirability.
Many years ago on tour, I visited Robertson and Sons. I believe they had the biggest bass inventory I have ever seen. This is included a sizeable collection of fine quality basses. I found I preferred some of the Shens that were priced around $6K-$7K over all of the fine instruments I demoed. The Shens played easily and had a mix of tonal qualities that I like. The store also had one hybrid that I found to be remarkable. Not sure if it was a Thompson RM200 or a Shen. I felt it was about 80-90% as good as the Shens I liked.
So here's where I think it gets interesting. There definitely is some variance in quality with so-described "student" instruments as I shared my thoughts on above, but I believe that price and quality are pretty strongly correlated up to a certain point. This correlation continues into the higher price tiers but I believe it gets more and more nebulous as we venture farther past the point of diminishing returns. My Shen, at least to my ears, has compared favorably to many (but not all, certainly) instruments costing multiple times what I paid for it. I have played numerous basses, almost all of them old, that people were asking anywhere from ~$10K-$25K that simply did not sound or feel as good to play. Maybe they just needed new strings and a setup, or would have sounded and played leaps and bounds better than my Shen in a section. Who knows.
Certainly, though, I have also played fine basses that were no doubt worth the many thousands, sometimes even six figures being asked for them. Truly marvelous instruments. I maintain, though, that even at that tier, price does not always equal quality. It's kind of like wine; there's a lot of hype and it's easy to get pulled into it if one isn't careful.
Regardless of price point, listen with your ears, not your wallet - easier said than done, I admit!
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, they may think of laminate as possibly a crude beginner instrument 
. This level of shop would most likely rent out laminate basses, but try to talk anyone who was serious enough to want to buy their own into considering a hybrid or carved instrument.
Also, crappy instruments are not student instruments. Parents don't want to deal with an instrument that's unplayable, or falls apart, nor do schools or rental shops. If it were possible that they could just buy an entry level outfit from an absolutely trusted brand, sight unseen, they'd do it. Yamaha has that reputation.
in essence, caveat emptor.