The school teacher bought a mobile phone detector and didn't know how to avoid being found.

It is impossible to find out who is using it. It is unrealistic for a teacher to carry this instrument around. The principle of this detector is to detect by electromagnetic wave induction, which is the same as the electronic dog in the car. Every once in a while, the mobile phone will send a signal to the nearby mobile phone base station, indicating that my mobile phone is on and exists. The mobile phone base station will also reply that the channel is ready for you, and you can use the mobile phone at any time. The mobile phone detector detects whether there is a mobile phone by sensing whether there is a signal of this frequency nearby.