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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Non magical people living in the real world. It’s you. You’re a muggle.
You don’t know that
I do know it, via magic. You can’t prove I don’t know, because you’re a muggle.