Remember a few months ago when a high-frequency alarm system made the news, as a means of deterring teens from hanging around? Well, some genius has realised that the same sound can be used as a mobile ringtone that can't be heard by adults - in the classroom, for instance.
http://www.vnu.co.uk/vnunet/news/2156885/teenagers-hijack-silent-alarm
http://www.vnu.co.uk/vnunet/news/2156885/teenagers-hijack-silent-alarm