This question comes up from time to time and I almost invariably recommend the rue du Commerce in the 15th. I still think its a great shopping street, but there is a new phenomenom (sp?) sweeping les teens parisiennes and its called Brandy Melville.
Located on the rue de Rennes (another great teen shopping street) in the 6th, this shop is the IN destination for local teen girls. I was there yesterday and the place was mobbed, even though nothing was marked down for Les SOLDES.
As my girls waited in line to pay, I stood outside. I was not alone. 4 other Moms had had the ingenious idea. They had come from across the city to take their girls shopping here. And there was a line of about 100 customers just waiting to get in!!!
The quality is not great, but the style is cute and everything is one size fits all.
The prices I noticed were; skirts 30€, tank tops 15€
A tip... the line for the dressing rooms can be nuts. We went downstairs and the girls tried on their clothing in a discrete corner.