This is an old story but there’s a chance you missed it so here I go again: To stop teenage loiterers, Welsh inventor Howard Stapleton invented a device that emits a very high frequency sound that older people simply cannot hear. The tone sounds like a mosquito buzzing in your ear and so was called The Mosquito. For more about the science and the story behind the invention, check out the very good “Wikipedia page”:http://en.wikipedia.org/wiki/The_Mosquito on the subject.
Of course no good deed goes unpunished and the teens have struck back by using the same frequencies as alert tones on their mobile phones, thereby avoiding turning their phones off in class. A minor victory, but still an annoyance for teachers I’m sure.
If you feel like trying out the tones for yourself, the best place to download them seems to be “here”:http://www.freemosquitoringtones.org/. You can download a whole range of frequencies and use the highest one you can hear. Be warned though, one day that frequency will fall away and you won’t hear your phone anymore.