What to do?

I saw something disturbing. I don't know whether did I see wrongly because it was all too fast. When I was taking out mattresses, 2 boys was fighting(most probably). When I turned around, I saw that teacher slapped one of the boy's face. I was shocked that she actually did that. I should tell the manager but I don't know how to tell her. I'm scared of telling because I don't have evidence. What to do?