While the teacher definately should be punished, I’ve always thought that in cases like this the boys should be punished in some way too.
I will bet you that they were willing participants and loved every minute they could have with this teacher.
She may have been using them, but they were using her back.
They might like some cigarettes, alcohol, and drugs too but it would likewise be criminal for her to provide a 15 year old and 17 year old with them.
The teacher is expected to be the adult. People in a position of power are expected to set and enforce standards.