I didn't get a really aggressively stupid teacher (or, at least, one I realised was that stupid) until I was 12. I worked out (I think) 37 times 4 in my head (it was thirty-something times an integer less than five) and he looked at me like I was a suspicious freak who was up to something for having been able to do this in my head and not consider it remarkable. I shudder at the thought that that was ever in charge of children.