250 to 300-watts is probably about right for most Jazz Band uses. That along with the ABY switcher would be a good choice. Probably best to check with Rumble by Fender or Rocket by Ampeg. Both have similar offerings and are reasonable prices. Asking for a school discount may get them down within your budget or check directly with Fender and Ampeg to see if either of them offer a school discount directly. Last time I looked they were both around $629 to $649 new. School discount might get you down lower. Either of those would have enough watts for most things (350-watts rms for Fender Rumble 500 and 250-watts rms for Ampeg Rocket Bass RB210). If you add a second cabinet to either combo down the road then it will use all 500-watts to drive the cabs (250-watts each).
Be aware that most (not all) combos that are listed as 500-watt combos are rated for 500-watts at 4Ω. The internal speaker(s), with few exceptions (such as Eden combos), are 8Ω speakers so they only get 1/2 to about 2/3 of that 500-watts as they come. When you buy a second 8Ω cab to go with the combo, that drops the ohms load on the amplifier to 4Ω and it will deliver all 500-watts to be split evenly between the cabs. Nonetheless, 250-watts or a bit more into a 210 cabinet is usually enough to play small and medium-sized venues. To play larger venues, you add a second cab which gives you not only more volume because of higher wattage, but also because you've doubled the area of speakers that are pushing air.
