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.
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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.