I usually don't fall for assholes, but..?

How can I stop liking a guy? This might sound stupid... But I really need to stop liking him. I always tell myself "From today, I won't like him anymore" and it actually works.. but then I see him and I can't help myself. It's hard to avoid him, he's in my school. I want to stop liking him because his personality is freaking crappy. He's really immature, I hate that, he laughs out loud, screams as loud as he can, says inappropriate jokes etc. Plus, since he came in my school, I'm really bad in school and my grades got worse, I even failed many exams, which I never did before he came. He's not in my class but it's impossible not to see him on the hallways, I tried to go to places he never goes to, but I always end up seeing him...I feel kind of shallow.. He's really good looking, many girls like him and he's always surrounded by girls. Once he got punched by another guy, and I felt really sorry for him, I wanted to beat the crap out of that dude.. but in the same time, I really hate him sometimes.