I, too, would reiterate to her that if she desires more freedom and priviledges, she will earn it, age-appropriately, as long as she is trustworthy and responsible. And adding something like...."...Now that it is getting warmer, I just want to reiterate to you my rules of walking home......" If she DID lie, and she has a conscience, she'll be dying inside and saying to herself. "How does mom KNOW these things???" (I don't ever reveal my sources.) Peer pressure is tough. My children know that, if they have done something wrong and they tell me before I find out , they will get in less trouble when it comes from their lips. When children tell you something unflattering about themselves, it truly shows courage on their part and tells me that they have a moral conscience. I just dealt with this with my son... I watched him squirm and get upset as he was telling me his story. It's a little heartbreaking. I stayed calm and he did see that I was upset. I still thanked him for for his courage. He still suffered some consequences (which he expected) but he knows not nearly what he could have endured.
There are times when I have suspected something in my children, and with my limited knowledge, have asked the question, "Is there something you need to tell me about...?" my daughter usually pours her heart out... my son is mumm until he's ready to fess up. I learned this from someone, long ago, and it is amazing what they will share. Also, practicing what you preach and not allowing yourself to be involved in "little white lies", sets the right tone.
Communication. Communication. Communication. And kids need to know that it is important for them not to break that trust that you have for them. My children have also challenged me lately with letting them have a little more freedom. This has been hard but I've been carefully giving it to them. Most importantly, they need to know that, no matter what they do, they can still come to you when they mess up or need advise, knowing that you are going to handle the situation reasonably and their wrongdoing won't be, continually, thrown back at them, in a heated moment. If your daughter doesn't fess up, I would let it go. You've set the tone now. There will be other battles to fight.
Good luck.