How to stop sexual thoughts?

Alright, so there is this guy in one of my classes.
Some would use the term “hallway crush” to describe our relationship. One thing tho is that i cannot stop thinking about him sexually, sure i think about him romantically but the sex thoughts do not stop. I want to pleasure him, make him feel really good. How do i stop these thoughts? especially in school. Should i finally approach him as a friend and see where that goes??