I'm not going to let my girlfriend be friends with my classmate
Why won't you let her be friends with your classmate?
I want her to make her own friends.
You shouldn't tell your gf who she can be friends with or not. It's her business and not yours.
I don't trust him. He's a loser and she shouldn't hang out with him.
Well, just because you think he's a loser doesn't mean that she will. You should let her make her own choices and not control her every move.