Techno-savvy schoolkids in the west of Britain (Wales, to be specific) have inventively adapted a security technology to their own purposes. The Mosquito technology was developed by a UK firm to use high frequency sounds to drive young gangs away from shopping centers. Once people are over about 20 years old, their hearing is no longer very sensitive to the upper audio frequencies - a natural condition known as presbycusis, or age-related hearing loss. So the security concept goes, play loud, annoying sounds in the 18-20 kHz range and kids will move away (and dogs!) while adults will be not be bothered unless they are so close to the sound source to make it loud enough to overcome their loss of sensitivity.
So, that brings us to the teenagers' exploit - record the Mosquito tones (clever marketing name, that) and use them as ringtones on their mobile phones. Now they are able to get text message and call alerts during class without their teachers hearing them! News article here.
(Hat tip: GeekPress).