My daughter has this same issue! I think we've had a conversation about this already, when I was coming to grips with her celiac disease diagnosis. She is short and slim for her age -- only 4'7" and starting grade eight in a month -- and, as it happens, is also very young for her grade, which doesn't help. While all of her friends will already be 13 when school starts, she'll be 12 until the end of November.
Anyway, yes, this is tough. She is very self-conscious about being tiny. For things that have to fit, Iike jeans, I usually go to the Gap or at a cute little boutique shop in the mall that sells jeans and yoga gear for tweens. I can't remember what it's called, unfortunately. The Gap and Old Navy are good overall for age-appropriate clothes for small tweens and teens. I've found that clothes from Old Navy are more likely to fall apart, however.
For leggings, shorts, tank tops and sometimes hoodies, I order from Hanna Andersson in the U.S. Don't be put off by some of their incredibly dorky clothes for little girls; they have some excellent basics and the quality is superb. My son lives in their clothes (they boy stuff is better, design-wise, than their girl stuff). I also recently got her a cute pair of suede "clog boots" from Hanna, which she fell in love with and begged me to buy. They make her a couple of inches taller, which makes her very happy.
For modern-looking tops, I've occasionally and success at Aritzia. They make a lot of tops that are meant to be loose and flowy, so they don't have to fit as precisely as bottoms, and they do a lot of ridiculously small XXXS sizes.
Hope that helps!
