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.
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My wife, who is a big potter fan, told me they take traditional classes as well, but they are shown off screen as they aren’t interesting to the story.
Pretty sure they explicitly talk about those kind of classes in the books. They don’t have scenes in them, but I remember Ron and Harry complaining about arithmetic, or mathematics, or something like that.
“Arithmancy” is what they call their math disciplines and it’s mentioned a fair bit in the books, though understandably never really shown in the movies.