This was before my generation, but apparently nowadays, mobile phones in classrooms seem to be a problem. Therefore many teachers prohibit their students to have their mobile phones switched on during classes. Obviously, those who ignore these rules will have to take pedagogical actions. Anyway, a british student now invented a ringtone, that cannot be heard by adults.
It works due to the fact, that the spectrum of audible sounds in adults is more narrow than on teenagers: Adults can usually hear up to the frequency of 14 kHz, whereas young people are able to hear sounds of 16 or 17 kHz. Therefore the ringtone can be noticed by the students as a very high pitch sound.