a little help??please??

Well, he probably acts like that at school because he is insecure- he's been bullied before, and may be being bullied now, so that's no surprise. 你 say he's nice outside of school, and he seems to like you, so why don't 你 ask him if everything's alright at school and stuff- just talk to him, and see what he's really like. It's most likely that he's a nice person who just has problems at school.

Hope that helps!

I have the simpliest, and most likely scariest solution: ask him.
Tell him to stop 表演 like a jerk.
In my opnion, two wrongs don't make a right- so, just because he was bullied, doesn't mean he should bully others.
I think he DOES actually like you, if he's trying to impress you, but at school, well, 你 know how it is, you've got to be "cool" and the guys HAVE to act all tough, like, "Oh look at me! Look at me!" And if he's seen being nice his 老友记 will probably pick on him so he's just trying to avoid it. Also, people act differently in school naturally just because there IS others around, some who like 你 and some who don't, it's like trying to survive so that they won't make the rest of his school life a misery.
He knows you, right? So, he is most likely 表演 normally around YOU.
But I honestly just say ASK HIM. It WILL be scary but if 你 get a good answer, then no harm no foul and if 你 don't get the answer 你 want, 你 can 移动 on and just act civilised because he is you're friend's brother.