How *proof are they???

I bet a determined teenager can try water, different kinds of soft drinks (acids and sugar, kids), different kinds of easily available solvents, different kinds of available oils, different kinds of hammer hits...

And I suppose enveloping it completely with magnets so that the magnetic field completely surrounds it might be enough to interfere with the signal at all times, but wouldn't render it useless and is likely to be examined at school once the absence of signal is noticed...

The point is, if you want to mess with an electronic device, excuse me but that is a piece of cake...

Of course, the most well determined and learned students would be well-advised to try the two ultimate bests: pass them through metal detectors likely to corrupt the batteries; or build an EMP device and render them a nice work of jewellery completely useless for any electronic purpose...