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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Reading is very common at Hogwarts and the wider wizarding world. Hogwarts has an extensive library, and most of their communications and news media are done over written mediums.
Contrast that to maths where the hardest thing most wizards would realistically have to do is basic money operations. Made even harder by their bonkers monetary system.
I think the OP meant that if they all stopped learning maths at 10, and there was no English class at Hogwarts, they probably stopped learning to read at 10 as well (well, the Muggles anyway, there’s no mention of how the Wizards learn anything before 11 I don’t think?).
There wouldn’t be additional English literature classes, sure, but by age 11 it’s mostly just practice and looking up words you don’t know anyway.