"No thanks, I don't really drink." Bart said honestly and without judgment. He didn't really approve of everyone else drinking and he felt a little squicky about not pointing out that it was illegal and all that... but he didn't want everyone to hate him either. Nobody wanted to be the That guy at a party; not even Bart... especially not in front of his girlfriend.
So even if it was wrong, and he knew it was wrong... he just wasn't going to do it himself. That was still doing the right thing, wasn't it?