It's scary to hear that sending text messages on mobile phones in school exams will be detected by teachers. Is there such an instrument?

It is an instrument similar to an electric mosquito swatter, called a metal detector, which will detect the mobile phone according to the situation. If the invigilation is not strict, you put the mobile phone and the key together, and the key will be taken out as soon as the machine rings. Or put the mobile phone near the belt, and the metal on the belt will also make the instrument sound, so the invigilator can't help you tie the belt.