It looked more to me that virtually all of Yuuta's classmates were giving their (at the moment, unspoken) APPROVAL to "the possibility of something happening" between him and Mary-sensei.
"A blessing in disguise" indeed. Even the stuff he was imagining on the way there wasn't exactly...Off the mark.
I almost didn't notice that he was having anal sex with her because her reaction suddenly changed the second the glasses came off, so I thought it had something to do with it.
Great ending though. Honestly, after something like that, I'm not sure why you'd bother 'studying your ass off', though I guess in that sense, the private lesson worked wonders. Heh, besides, a little bit of extra supplementary/private lessons is always helpful.