He was abducted by Hagrid when he turned 11 so that would place him maybe around the fifth or sixth grade.
I don’t know if canonically there are math classes at Hogwarts.
The thought came to while I was watching the anime Mashle. If you are into Harry Potter and One-Punch Man I’d recommend giving it a watch.
Someone mentioned this community below; I wanted to highlight it.
Small promotion for !harrypotter@literature.cafe
That’s because Mudblood is a magic user born from a Muggle family. A different term from Muggle itself. Pretty sure Muggle is an accepted term in the overall society (it’s used in the Ministry’s Department name for example), except from a few ‘activists’ you might hear of.
I believe there’s someone like that in Hogwarts Legacy as well.