I love that Alastor bet on him, and that he wondered why Rufus didn't look happier about the sort. I like the structure of the school that is described, with the firsties being talked about like livestock.
contentious as pit full of bears This is an amazing bit of description. Rufus, previously interested in watching songbirds, found it quite agreeable to watch Alastor Moody. And I love this sentence.
Rufus felt his mouth move and marveled at what came out. as Gryffindor had bled points, Slytherin had positively hemorrhaged "He does not." The idea was ridiculous and a bit obscene. He wasn't a kiddie-diddler. Alastor Moody was a pain in the arse, not so much hard of hearing as he was hard of listening, and Rufus often heartily wished a variety of discomforts, inconveniences, and indignities upon him. He would be heartily ashamed of himself, he knew. Later. Maybe. whilst literally half-arsed.
I like your writing style. There's not anything I can add to these parts except that I like them.
For shit's sake, you think I've never seen bums before?" This was the most hilarious line.
Rufus might not be a romance novel cover, but he was easy on Alastor's eyes. I think this line encompasses the exchange.