Ok, first thing first.... It's going to be very difficult on you since you like a friend's ex. Were they really close? Was it a really bad breakup? && who broke up with who?
Depending on how close you are to you friend will determine if she'll be ok with you liking him. But hunny, remember this, it's perfectly OK to like someone! If you're holding yourself back from someone you really, really like, then you are denying yourself happiness. It's unfair to your heart.
Relationships take risks. You gotta put yourself out there in order to see if they have the same feelings. But guess what? If you don't, then you'll never know how he feels!
Does he drop hints that he may like you?
&&, above all, just because 'every girl in school' wants him, doesn't mean he wants them. I think people tend to forget, that it's the guy's choice too on who he wants to be with, not just the girl's.