There may be a lot more going on than you know. If she doesn't follow dress-code rules she disagrees with, it's very unlikely she follows other rules she doesn't like, and she has unsupervised time in which to do so. She's proven that she's susceptible to peer pressure, that impressing her friends is more important than anything you think, and she's already lied to you many times.
Based on what you've written, I would be very worried. It's possible that the only misbehavior involves wearing short skirts, but that's not realistic, is it? Unlike Dan, I wouldn't trust her, but you don't have to say that. Too much is at stake - this is not a dispute about community standards among rational adults!
She's fifteen, and will be driving soon if you allow it. What will she do then, when school is one direction, the mall is another, her boyfriend is horny and the house is empty? Am I concerned? Yeah. Am I being overly alarmist? I don't know. Do you? If I'm wrong, you need to know it, not hope it. I've taught over 5,000 14 year old kids in 29 years, including both my daughters and most of their friends, and I've seen this same story have a bad ending.
If it's at all possible, you and your husband should shift your work hours so that one of you can be there when she leaves and one is there when she returns. If you can't, get as close to it as you can. Her unsupervised time is a problem. Supervising her is the solution.
One last thought: When I was in high school, my friends with the cool parents became the first parents, except the one who died driving drunk. Teens look, and for short periods, can seem like rational young adults. Don't you believe it! We weren't, they're not. They're emotional creatures, living for today.