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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I’m just over here chuckling at your casual statement that Hagrid abducted Harry.
He broke down his door, mutilated his cousin, threatened his uncle, then lured Harry away with cake (which was eaten by said cousin).