They were not teachers. The relationship between a teacher and student is far different between an older man and young girl.
Society expects for a teacher to take care of its children, teach them and protect them. Students are not capable of making decisions on their own and it is a teacher's job to make sure they can. Until they graduate they are not fully functioning citizens of the community. Any mistakes that should occur it will happen under the watch of a teacher to make sure nothing bad happens.
Now if a teacher should get involve with a student, this violates that trust between parent/society and the teacher. The teacher took advantage of his or her position. A student can fully argue that he or she did not know better because the teacher allowed it to happen. They are guardians and they are expected to protect you. And then they harm you, they who is there left to trust? If an older, wiser guardian/mentor can do it, then anyone can.
I really can't describe the morality of it, but there is a reason why student-teacher relationships are far more taboo than older-younger relationships.