I think that the most important things about a girl's clothes are that they are neat, clean, and high enough quality for the occasion.
Neat: If your clothes fit your body style and personal style well.
Clean: If your clothes don't show the residues of today's lunch, and they are ironed properly.
High Enough Quality for the Occasion: The brand name doesn't matter, as long as the clothes are appropriate to whatever occasion you are in. I know girls that can pull off looking like stunners in $8 outfits from the thrift shop that are carefully selected and put together well, and I know girls that look like ass in their latest tacky $700 purchase from Nordstrom's.
It doesn't matter if the brand is designer, it just matters if it looks good. You're under 18, so other than like Abercrombie, Forever21, and Victoria's Secret, no guys are going to know about women's clothing brand names anyway.