I don't know if you're now leaning towards boarding school, but I live in LA so I can tell you a little bit about the schools you mentioned:
Harvard Westlake is very rigorous academically, and when it gets to high school they tend to choose students who are exceptional at sports, music, art etc., but I have one friend who got in for junior year on his superb grades alone.
Brentwood is a good school, and I know people who have gone there and been dissatisfied, as well as people who have loved the school. It's kind of a toss-up. They have a beautiful campus, though, with excellent sports facilities.
Marlborough is also very, very academic, but I know plenty of girls who are happy there. I hear it's a warm community, except for the fact that they get a crazy amount of homework each night.
I don't know if you've looked at Archer, but it is a superb school. It's right across the street from Brentwood, and it's all-girls like Marlborough. They have very small classes (I think the average is around 14) and it's a great community. They make learning fun for the girls, and the sports programs are getting better and better every year. They do famous plays and musicals, and their orchestra and choir have won countless awards. I'd encourage you to go take a look.