Okay then. Hmm. Well then, him being her old ex-boyfriend changes things. Hmm ... okay then, I like the reason you give. More to it, I can relate to this because it happened to me and my bestie when we were still high school. Only, the roles were reversed. And speaking from the perspective of your best friend, I'd tell you that yes, you should tell her before you ask him out. She would appreciate the honesty. Trust me, she would.
I'm was going to through out the juvenile excuse of, "because we're best friends," but then I think I'm going to decide on, "because we're supposed to tell each other everything because she talks to me about guys, so why shouldn't I do the same to her?" Plus, she might have some insight, considering it's her really old ex-boyfriend. Old as in they dated a while ago. A while ago being...five years ago.
I've never understood that concept. Isn't it your life? And your risks that you are taking by asking him out?
If you like him, go for it. If he says yes, then that's awesome. I don't think it's weird that you would find feelings for someone in home room. I've had HUGE crushes on guys three years my senior in high school.