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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What they lack is knowledge about volume and surface calculation. That is some stuff you definitely need in life. That wizards don’t need analysis or Calculus is kinda obvious.
For trivial calculations that’s still going to be accessible just by looking up a formula. For more complicated ones… I can’t remember the last time I needed something like that. What sorts of use cases are you thinking of?