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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Just a warning: this takes pretty much every bad FF trope you can think of and turns it to max. Awful reading.
I stopped reading it halfway through, and was too lazy to figure out why.
This explains it.