Hi I am considering a similar problem. We had said months back that my daughter 17 could go to Spain for a week with her friends if she funded it herself (she has a part-time job) and organised accommodation we approved of. A friend's mother is prepared to book it for them. Thre seemed to be no reason why not.
I know from what she told me of her holiday in the UK with the same friends last year that they go to bars/cafes and pick up boys and then run off when it gets too much. She thinks she can cope and to some extent she can. My feeling was/is she has got a good sense of self-preservation - she just doesn't choose to prevent some things from happening to her. She does tell me things, but she also tells me lies. Depends whether she wants support or to hide something.
Then last week we left her for 2 school days with a family friend, but she asked to sleep at our house to be ready for school. We came back and found she had made plausible excuses to our family friends, gone out, lost her important coursework, got drunk with friends, skipped off all school, lied repeatedly to everyone and made herself ill with asthma by tiring herself out.
Of course we found this out - her stories never make sense so I asked school. We have said she can't go on holiday now (and she has to immediately catch up on the school work over anything else).
Before, I thought you had to give reponsibility to teach responsibility. Now I think it is only this way that she will see that there are consequences when you let others down and dont do what you were trusted to do. She is furious and wants us to agree another punishment/outcome but I am not going to discuss this. Otherwise I am just waiting for someone else to hurt her and teach her the lesson. I am kind of unhappy to set a punishment that is a couple of months off, but frankly she isn't sensible enough to go. Its more that we have changed our minds about the chances of her growing up enough than that we want to punish her.
Thanks to this message, I will put her passport somewhere safe and make sure the friend's mother knows that she cannot go. At the moment I could not trust her not to get the holiday booked for herself anyway.