I wish I had the mind of a teenager.
Compund Security Systems have developing an alarm that can only be heard by young people under the age of 20. Capitalising on a medical phenomena where we stop hearing sounds at a certain frequency (18 to 20kHz) after the age of 20. The intended use was to push the kids out of malls and places where they may be a nuisance (a worthy cause of-course I mean kids should be treated like insects right?) So here we go the kids record the alarms turn it into ringtones to pass on to their friends so that when they call each other in class the teacher can't hear them. Brilliant. More here.