I have the same lip from my 9 year old too. Lots of 'whatever', 'bothered' etc!! My son is an angel as a rule and i am forever complimented on his behaviour. Back at home, life is rather different. If i tell him to go to his room he just says 'make me'. Nightmare! Don't know how to respond to this at all.
I have threatened to take him to the head teacher the next morning or the doctor as his behaviour is unreasonable and mummy needs help. This plays on his conscience for a while.
Not much help i'm afraid but at least you know someone else is suffering!!!