I never really went out during my senior year (last year), just because I was so busy with everything. I was driving to a town 60 miles away for an area band and orchestra three times a week, and I had multiple extracirriculars that sometimes led past 8:00. Maybe she is wanting to go out later because all of her friends have some sort of sports practice that lasts until 6 or 7. That would mean that she would have only 2 hours with her friends, less for traveling time.
I believe the OP is being a bit unreasonable, and it sounds somewhat like a power struggle for independence. Maybe the OP should try giving her daughter a responsibility (say, an extra chore) along with the privilege of going out maybe 30 minutes later or an extra night, instead of just saying "because I said so." (which seems like it is worsening the relationship between the OP and her daughter) It sounds to me like homework is not an issue, maybe she's finishing it at school (that is what I did), and I got good grades through high school as well.
To me, that IS going out on school nights. Her Dd wouldn't be able to do any of these things according to Mom's rules.
My kids know what they need to do for homework and how long it will take them to get it done. If their grades are good (A's like usual) then there is no reason to restrict what they do in the evenings. They also know how much sleep they need and know when they need to be in bed. They have wonderful time management skills because THEY have been able to figure out on their own how much time things take to complete.
But seriously, that's not why I limit her. Just because I'm single doesn't mean I want her to keep me company. I DO have a life...I take kickboxing class twice a week and I get up at 4:30am for work so I go to bed early. During her sophmore and junior years, her grades were not good at all. As a matter of fact, there was an F on her report card and numerous Ds. I assumed it was because she was doing too much. So this year I put a limit on it and it seems to be working. Now of course, school activities do not count. And when I say out, I mean hanging out at friends houses, movies, shopping, eating...

