Couldn't he have placed the microphone/transmitter among her school stuff back home? It could have perhaps been done in a way she wouldn't notice even afterwards. Now she might recognise the thing doesn't belong to her when she opens her bag after school or even during the school day. It would be a creepy discovery. He would still need to infiltrate her school, probably, as bluetooth or wifi don't necessarily have that much range. If such an eavesdropping device can be found equipped with a 4G connection, it would solve those problems, naturally.