Well, being a teacher I had one experience of my private student just turning round and kissing me at the end of the lesson. She was 26 years old!
But if I had to choose, a fantasy is a student stays behind to talk, everyone has gone, she asks for help and she is close and bends down while I am sat at the table and reveals her breasts. She looks at me, and says sorry but has a little smile and moves closer.
