I have to say, this is really clever:

A high-pitched alarm which cannot be heard by adults has been hijacked by schoolchildren to create ringtones so they can get away with using phones in class.

The alarms (called Mosquito) had been installed by British shopkeepers to keep rowdy teenagers away. Now the ringtone is spreading virally via Bluetooth and text messages, and the kids can receive phone calls and text messages during class without their teachers hearing anything. The law of unintended consequences folks, it’s a crazy thing.