He pokes me and PULLS my HAIR!! WTF?

It started a week or so after we moved seats, and at first I like didn't even know he was there and I'd talk to my guy friend all period. A few weeks ago, the guy who sits behind me started poking my in the back with a pencil, and after a day or so, he started poking me in the waist. I'm pretty ticklish, so I'd start laughing while telling him to stop. The day he started doing that, his old seat partner who sits at my lab table cuz he talks to much was like, "Greg, this is science, not flirting, see? I told you Nica!" cuz a week earlier he told me, "You know Greg likes you, right?". After that, he started trying to trip me in class, and he'd talk about how he was the TA in for the teacher so he knew everything from correcting homework. Today, the teacher posted our test grade, and when I checked it and came back I was pretty bummed out because I got a D. When I sat at my seat he started talking about how he got a B, and after I told him I got a D, he patted me in the back like 5 times, then started pulling my hair and poking me, and he wouldn't stop! My guy friend came in the room right then, so I was like, "Greg's harassing me!!" And my guy friend told him to stop and that only he could do that to me, and he stopped for a little bit. He started to bug me once AGAIN, and again my guy friend started making playful threats that we knew he wasn't serious about. We went to work at our lab tables, and when we came back before the bell rang he started poking me, and walked out of the classroom. He's been doing this for awhile, and I still don't get why he does that. I've known him for 3 years, and he never bothered me much before, so what's up?