As someone already mentioned, you really don't want to have to sand or file down a bunch of bridges to fit pickups.
I agree that buying a few inexpensive piezo "wing" pickups is the way to go. If they fit the wing, great! If not, get some four-for-a-dollar plastic clamps and just clip to the underside of the bridge. Will they occasionally move around or get knocked off? Yes, and then the students can re-fit them, and think about how and why the sound is being produced. Pic attached.
Also, I suggest adding this to your camper brochure: "Bass students who do not have a pickup can buy one at the camp." That way you don't have to worry about the pickups disappearing.
Cheers,
Paul
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