I don't feel safe

So at one of my classes there was a boy that came and sat two seats down from me. He kept staring at me and I already felt uncomfortable. I watch this kid put his hand in his pocket and start moving it around, still continuing to look at me. I made it clear that I knew what he was doing but he didn't stop. When we were dismissed I tried to wait for him to leave first. Being looked at from behind makes me feel very uneasy because I have a very large behind. However this kid wasn't leaving class so I just left first. He left immediately after me and walked very close. This has happened twice. Is it safe to assume that he is doing it purposefully and how do I go about making it stop? I've been in situations like this before and I was told to "get over myself. Not everyone wants to look at your ass" and my teacher is an asshole. SOS!!!!