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 think that’s a bit over simplified. Do you think they’d be able to understand Shakespeare? Or a spell book from the same time as Shakespeare? There’s a reason we continue to teach English until a child leaves secondary/high school.
The bigger question is, do they need to understand Shakespeare?
Shakespeare is important in UK Muggle culture, not so much elsewhere, including the wizarding world which very much has their own cultural icons.
They probably aren’t getting their spells from books that old either. The reason we don’t transpose Shakespeare to modern language is the loss of artistic intent in the process. Something that wouldn’t apply to purely factual books like a spell book.
Besides, they are expected to learn Latin; that’s where their spells come from.