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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Arithmancy at Hogwarts is also very different to traditional arithmetics. It is the use of numbers to predict the future, kinda like divination.
Yeah, reading the wiki, it seems like it was a lot of memorizing number charts.