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.

Techno-savvy pupils have adapted the Mosquito alarm, used to drive teenage gangs away from shopping centres.

They can receive calls and texts during lessons without teachers having the faintest idea what is going on

The alarm, which has been praised by police, is highly effective because its ultra-high sound can be heard only by youths but not by most people over 20.

Schoolchildren have recorded the sound, which they named Teen Buzz, and spread it from phone to phone via text messages and Bluetooth technology.

Now they can receive calls and texts during lessons without teachers having the faintest idea what is going on.