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.
A secondary school teacher in Cardiff said: 'All the kids were laughing about something, but I didn't know what. They know phones must be turned off during school. They could all hear somebody's phone ringing but I couldn't hear a thing.
'One of the other children told me all about it later. I couldn't be too cross, because it shows resourcefulness.'
The Mosquito technology is said to play on a medical phenomenon called presbycusis, or age-related hearing loss.
It is thought to begin at 20 and first affect the highest frequencies – 18 to 20kHz.
The device was developed by Merthyr Tydfil-based Compound Security.
Boss Howard Stapleton said: 'I think it is a giggle. A teacher would be able to hear the sound only from 1m away. Teenagers could hear it from much further away.'
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