Actually, I can hear it, but only just, at the very edge of my hearing.
It’s a sound that was invented by British shopkeepers to keep teen loiterers out, but is now being used by teens so that they can receive text messages without teachers noticing.
Even though I can hear the sound when I’m listening for it, though, I’m not sure I could make it out in a crowded classroom.
In the usual double standard way, while I’ve heard some indignation from adults about the teen call phone use of the sound, I haven’t heard any indignation about the shopkeepers’ use. Even though, you know, playing a high pitched tone to irritate young people into leaving your store isn’t any more polite than taking calls in class.
ETA: here’s the sound with crowd noise in the background. Much harder to make out this way.
Also ETA: A web search does reveal some irritation at the shopkeepers after all–and more amusement at teens turning things around, too–than I’d first assumed.