[fungoking]

devices that emit a high-frequency tone. .. generally only be heard by those under the age of 25
Where can I get one???

It's called The Mosquito, it emits a high pitched sound that adults can't hear. I'd play an extremely loud sample to my AP Psych students, I couldn't hear it but they would cover their ears. Business use it to keep teens from loitering.

I've found that classical music works well on most teens, country music works on skateboarders.

I had students use it as a ring tone. I couldn't hear it, but other students would complain about the noise.

http://www.freemosquitoringtones.org/the-mosquito/

[BBell]

I've found that classical music works well on most teens, country music works on skateboarders.

A McDonalds near me will play old country and blue grass. Works like a charm although I would prefer classical music.