I think I've got to disagree a bit with other people's responses. Personally, I think the best way to handle it would just be to say "Please don't lead me by my bra strap." End of story. Not quietly, not loudly, not in an accusing way, and not in a self-deprecating sort of way. Just neutral. That way, there is no confusion about what you mean and no way to think that you don't mind or are laughing it off (as could happen with a joking manner, if he's so inclined), there's no accusation if his actions weren't meant to be icky, and there's not even a hint of you taking even some of the "blame" for what went wrong. Don't make a bigger deal of it than it (perhaps) really is, but don't minimize it and sweep it under the rug, either.
If he doesn't get it after that, I think it depends on the vibe you're getting with him. If he's just clueless and doesn't know what to do, ask the teacher for guidance. If he's being icky, I'd say it again, louder, and perhaps talk with the teacher after.