A teacher having sex with any student is not okay. It doesn't matter if it was that student's teacher or not. There are enough people in the world for that teacher to have relations with. A teacher needs to live to a certain standard and one of those standards is not crossing the line with any student no matter how old they are. This includes a 24 year old teacher and an 18 year old student. Or a 24 year old teacher and a 15 year old student.