I would say that both of those corrections are reasonable. While it may be seen as a matter of pretentious middle class by the upper classes in some places, in common society it is deemed far politer not to say "what?" in response to a mishead comment. Mind you, I would never say either, preferring the rather vague "sorry?". As for can/may, I would make this an explicit teaching point - there is a reason parents of generations have picked up on it: it's because the words mean different things. Can I go to the toilet? = Do I have the ability to? May I go to the toilet? = Will you permit me to? The interesting extension (that most parents/teachers miss) is how correctly to ask if someone else is willing to do something for you. e.g., Can you cut this card for me? I would prefer "Would you...?", but some may prefer "Will you..."