My ex boyfriend did that to me. I wasn't even wearing "sexy" shorts (notice the "s around sexy). He was just following me in a weird way, I kept side stepping thinking I was getting in his way or something. Then I felt him touch my butt. After that due to my fast reflects I slapped him and kicked him in his guy parts. He fell to the floor, I felt really bad but I laughed like a high person. But he learned not to touch me without me knowing. XD. Maybe try to stand in front of him and act like his being the weird one...
OK, since journalism is one of your electives in school, tell him that you are interested in writing a story about spanking: your theory is that boys don't like to spank girls, but you need to research the matter.
That should do it.
It'll help
If you tried that allready then try teaseing and flaunting your butt.
If that doesn't work, then he probably wont every really be into it.
Just ask babe.