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There is a nib/nipple (whatever you wish to label it) on the bottoms of the plastic valve guides that fits into a drilled or partially-drilled hole on the tops of pistons (for alignment).

The nib/nipple isn't a super-tight fit into the hole drilled into the top of a piston.

As your fidgety students (as you mostly are teaching woodwinds how to play fast, and baritone/tuba students are sitting in the back, bored) spin the finger buttons, they also loosen the stems. This allows the plastic valve guides to move subtly, yet still within that drilled spot on the top of the piston.

Good-condition guides can move barely outward too far, and drag in the guide slots.
Worn guides (which are just good enough to function) will retreat inward, slip nearly out the valve guide track, and the piston will stick.

REALLY loose valve stems will cause the guide to be able to spin and (rather than the drilled spot intended) the nib/nipple will land in the OTHER (larger) hole in the top of the piston - the vent hole - and the valve will spin.

If you can diagnose these problems, address them, and/or stop the fidgeting (maybe, figure out something for them to do...), you won't be sending instruments in for repair that might not actually be broken...or (when ten seconds of stuff will make them play again) setting them up on the top shelf for the rest of the school year, and - as a replacement - handing a kid a leaky "old dog".

Also, school-owned low brass instruments never end up being oiled, end up full of lime, and freeze up...OR they are oiled regularly, and the pistons are dropped onto the floor and bend...so (maybe?) have the students oil down the mouthpipe tubes towards the valves and wiggle the valves. It's a crappy way to oil valves, but the pistons won't fall onto the floor.

Never mind the "At my school, we always..." This for the others "who never".

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related topic:

There's also a way to get totally worn-out guides to work again (in an emergency). It requires that someone have some hand skills, decent close-up vision, and some attention to detail. I'm not going into that stuff, today.

Bloke, are those plastic top of the valve guides a standard part? What I mean is, my Kanstul has them. If I ever need one, will the Yamaha ones work? Or am I doomed because Kanstul used a proprietary part? Not having a problem with them right now, I just want to know where to go for the part if I ever do have a problem.

I've seen Yamaha-shaped ones used on Weril and Jinbao.

As long as they stick out far enough to catch the slot, they can be trimmed back with a file and a whatever if they stick out too far. There are also other little tricks such as barely shaving off the front edge or the back edge of the nib.

Jupiter ones have a slightly different relationship between the tip and the nib, and are worth a try with off brands and makes which are no longer made. I could possibly imagine that California manufacturer not wanting to invest in having some plastic fabricator make guides for them, due to the quantity that a place such as that would require ordering, so it wouldn't surprise me if that defunct California manufacturer bought theirs from Brazil or Japan. Jinbao doesn't seem as willing to sell parts, particularly if they're selling them to another manufacturer.

For what it's worth, I never liked any of these, and I don't like the system. The valve threads usually end up being about 5 mm diameter, which is probably a little bit too small to be strong enough to resist breaking, and I don't like the fact that stem tightness effects guide position. I also thought they messed them up when they started sandwiching in metal. It's not the valve guides' fault that children trial and error pistons into casings - beating the plastic tips against the tops of the casings until they finally manage to randomly hit the valve guide slot. The sandwich ones don't work as well as the all plastic ones, really become problematic when the plastic tip breaks off leaving only the metal, and they are just about as noisy as brass ones were.

Any manufacturer that uses this system should be making their stems out of stainless steel.