Why do teens dig their own graves

We have had a teenager for a week now. We set the expectations. Everyone was in agreement that it was fair. When asked a question, you answer openly and honestly so we can best evaluate the situation and come up with fair and reasonable choices. 

  Due to former habits and examples, we get half truths with some lies sprinkled on top. How hard is it to say, we want to go out on Lake W with friends, in their boat. Instead of, we want to go swimming at some lake around here and a few girls I don't know are going to meet us. Playing the dumb card doesn't work well when you spend so many hours with someone planning outfits for school 2 months away, but you claim to know nothing about the next day's activity. Um, your friend just filled me in and was honest with me and was even willing to accommodate and adjust, but you blew it. 

Just keep digging....