In my experience, if a teacher wanted to be kept reliably informed of subversive activity in his/her classroom they would always appoint a trusted girl. These angelic little little so and so’s, who usually had a schoolgirl crush on Mr Teacher, would happily “put you in” for breathing out of turn.
I mentioned elsewhere that in primary school a girl was appointed by the Deputy to inform on talkers while he was absent from the classroom. She would stand at the front of the room, surveying the assembled students, chalk at the ready to write down the name of anyone who so much as moved their lips. The offender would then receive one strike on the hand on the Deputy’s return – provided he was male.