Most Helpful Guy
This is why kids shouldn't be left alone together like that.
Technically, yes he committed sexual assault on you. Technically. I think most juries would struggle to convict though.
You're not over-reacting. You got a glimpse of how even a nice guy can be an animal thanks to hormones and opportunity.
Tell your parents what happened. Get used to the idea of letting them know if something like this ever happens again. And don't be alone with a boy like that in future, at least not until you're both much older.
Most Helpful Girl
You did the right thing. Your boyfriend acted in a disrespectul way. You told him to stop and he didn't.
You need to talk to your parents about this. To his defense, most guys during their adolescence can be extremely horny and naive. That doesn't justify what he did though. Teenagers of your age shouldn't be left alone like that. There should always be a parent present. Anyway, if he tries to do it again, then you better warn his parents and/or break-up with him. You are way to young to be in a relationship anyway.