This is the Teen Buzz ringtone -- most teenagers hear a REALLY high-pitched annoying sound, while most adults (usually starting from 20) hear absolutely NOTHING, no matter the volume.
If you hear it, this allows you to set a ringtone on your cellphone, without the teacher having the faintest idea what's going on.

I hear it, but "on the edge" (sometimes the sound seems to slowly vanish) and I am 14.
This one is more high-pitched and I can't hear it at all:
http://s3.amazonaws.com/listverse/audio ... hfreq1.mp3
Note: Please don't disturb yourself by trying to notice the sound if you don't hear it. If you don't hear it, you will NEVER hear it, no matter how much you try.
This works because with age, people slowly lose the ability to hear extremely high-pitched sounds. It initially was used as a teenager repellant in some shops, but clever teens found a way to use this to their advantages.
While I don't know why they just don't set their phone on vibrate, teachers, take care.
