I live in CA too, but I'm much closer to Oregon than Los Angeles.
My kids are/were homeschooled, low-income, and weren't bitten much by the clothing bug; I'm very lucky. My 14yods is perfectly content in a pair of thrift shop jeans and a plain black T shirt. My dd had some problems at that age which I didn't exactly understand; I thought that she was refusing to wear thrift store clothes for fashion reasons and it turned out that the thrift stores simply didn't have sturdy, practical inexpensive clothing in Junior size 1 and 3.
She had already begun buying her own clothes from Ross, Sears etc. with her birthday and xmas checks and babysitting money before I caught on. The clothing was poorly made and too revealing, but the designers are appealing to women/girls who have been on extremely stringent diets to get down to small sizes, not to young preteens/teens who haven't finished growing yet.
It's such a relief now that she can wear a 5 and not swim in it; I'll need to fatten her up a bit more to find thrift store clothing for her, but at least she can find decent styles.
She did not communicate this well enough at thirteen or fourteen for me to understand or else I was too stuck on the stereotype of clothing obsessed teenaged girls to listen.
Another incident I want to share that might give you some perspective happened to me as an adult:
I prefer to wear my hair long and untrimmed, do not wear make-up, and don't enjoy spending money on clothes for myself. Most of my existing clothing is older, fairly high quality, very modest, and rather "classic", because these are the styles my mother prefers and she likes to give me clothing for presents.
I was completely unaware of the fact until recently that people's first impression when they see me is that I "look like a Pentacostal". No offense to Pentacostals intended, but I'm not and I don't want people perceiving me that way! I fancy myself to be more of the "aging hippie" type and would probably have less trouble making like-minded friends and staying out of unpleasant political discussions with people of opposing viewpoints if my appearance were not so misleading.
Perhaps the different brands have similar social connotations at your child's school.
Or perhaps I am being an idiot and don't have a clue how lucky I am to have such undemanding teens when it comes to clothing and fashion.