"Yesterday you could've resisted me, but you didn't right"

Does that mean they did something???

I don't think they did it (from what it looks like). Maybe a bit of foreplay, or she stopped halfway? Or maybe they just cuddled all night long.

Really lets you wonder
The neckties came off, that's for sure
But you can't really tell what happened

Not a fan of the drunk, no resistance = liking/agreement line of thought, but ignoring that it's a cute story. I'd actually thought maybe they didn't do anything, because the freshman's just sitting there in the morning. True though that their jackets and neckties are gone, which I hadn't noticed on first read. And it would make more sense if they had been intimate in some fashion, even if it was just petting/cuddling, cause otherwise how could the girl expect the senpai to have made any sort of decision made when she was asleep.