Some students are using a new ring tone to get messages in class, and many teachers can't even hear the ring.
The ring tone is too high-pitched to be heard by most adults.
With it, high schoolers can get text message alerts without the teacher knowing.
The phenomenon is known as aging ear -- a loss of the ability to hear higher-frequency sounds.
It happens to many people as they get older.
One New York teacher says a colleague played the ring for a classroom of first-graders.
All of the kids could hear it, while the adults couldn't hear anything.
The ring tone is a spin-off of technology that was originally meant to repel teens.
A security company developed the tone to help stores annoy youths loitering in front of their stores while leaving adults unaffected.
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