My 5 yr old pre-k son hits in school. I have tryed it all. 2wks in the teacher ed him.he likes to say NO?

My son hits student tells the teacher no and rarely sees recess because of his showing out. i think hes doing it for attention. I have tryed everything i can think of. he is a bully and it scares me. He dont do it at home i dont get it. how do i stop it if i can't catch him in the act?