I believe the life of a teenager can be reduced to three sayings that I abhor:

1. "I know". They think they know everything and don't want to hear a differing opinion. So they ignore any input into their behavior.
2. "I was just....." This somehow justifies anything they have done or said. It is meant to justify #1 when it turns out poorly.

And then when you try to teach them you get (after arguing for a while),
3. "My bad". A very complicated statement in two words as it means that they are taking the blame (sort of), that the conversation is now closed for discussion, and all repercussions should be eliminated as it is somehow a pardon due to the admission.