Butt slap

For my whole 5 years in school studying Architecture, some of my straight guy classmates always slap my butt whenever I walk past them and it is so annoying! They are straight and I am very sure about it. They even stop and stare at my lips when I wet them or even secretly grind their dick(inside their pants) in my shoulders when I am sitting and doing school stuff. They always short-tickle me and laugh at me at my reaction. By the way, I let them do it to me because I don't want any commotion but except for the dick stuff. Is their behavior normal for straight guys?