A while ago, there was an ultrasonic sound going around called "mosquito" that was supposed to be inaudible to older people. The idea (at first) was to use it as an anti-loitering system by playing it loudly in places you didn't want those darn kids, like on your lawn. Then the whole thing got turned around and used as a ringtone that kids could hear but their teachers could not detect.
Now I got to thinking: it should be possible to rig up mosquito-detectors to let teachers know that their students are using the "secret" ringtone just by detecting the sound and flashing a light or something. You could probably triangulate the location if you used a couple of them together. Then again, it seems likely that the best way to detect a secretly-ringing phone would be when the student pulls it out and starts trying to discreetly text on it.
Mokalus of Borg
PS - Or talk on it.
PPS - Either would give you away very quickly.