You know what I hated? In school when you'd say "Can I go to the bathroom?!" And the teacher would say something sarcastic like "You CAN go to the bathroom, but MAY you go to the bathroom?" And then you'd have to say "MAY I go to the bathroom?" so you wouldn't wet your pants.
I'm glad that these teachers think proper grammar is important. But I can't imagine a more trivial use of language. I also don't understand how they are able to distinguish between Can I / May I but don't have the ability to determine that they shouldn't have sex with a 12 year old boy.