For the life of me I will never understand teachers having sex with students. 
No, it isn't supposed to happen.
But as a graduate of an all male private school, I can understand the students checking out their female teachers...BTDT.
Still, adolescent fantasies aside, it would not be so hard for a reasonably attractive female teacher to find an interested young man (16-17-18 years old).
And that's not even getting into the Professor/grad student things that go on. (I know, they are adults, but often there is some sort of adultery going on there.)