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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She successfully brewed a complicated polyjuice potion that took months to make by just reading and following the instructions in the book. She reads just fine.
But all wizard books are scribbled in crayon