Does the girl wear a uniform at school? If so, she can get by with less regular clothes.
If she wears regular clothes to school, she should probably have at least 10 different pairs of pants and tops, but they should be in colors/styles/fabrics that can mix and match easily, creating way more “outfits.” I’m assuming that she wears more pants than skirts, but if she wears more skirts than pants, then choose more skirts and less pants.
I never wore shorts to school, but if this young lady does, then add in about 4 pairs of shorts. Again, things should be easily able to mix and match with the tops.
What kind of climate does she live in? If it gets cold in the winter, then she’ll need a few sweaters, which I was already counting in with the tops. Maybe 4 or 5 different colored sweaters, but again, make sure they mix and match with the pants and skirts.
As far as shoes go, again, it kind of depends upon what type of climate she lives in and what kinds of activities she’s doing. When I was in school, I mostly wore tennis shoes or a pair of casual leather shoes, but I also had a pair of nicer shoes that I would wear to special events and I had a pair of flat sandals (but some schools don’t allow open toed shoes or sandals at school) She might need a pair of specialty althletic shoes depending upon what types of sports activities she’s involved in, including regular P.E. classes (the school might have a preference).
It’s better to have more bras and underwear than you need, than to have less than you need. They’ll last longer if you have more of them. So have at least 2 weeks worth and maybe even 3 weeks worth. Also, because she’s 13, she might have some little period accidents that need to be accounted for, so make sure there’s plenty of extra undies.
Belts, depending upon the colors of her clothes, she should probably have at least one black and one brown belt, but maybe a white one too, depending upon colors of the rest of the clothes.
Accessories completely depend upon the style of clothing and her taste and her budget. Maybe about 5 different sets of earrings, I’m thinking a good pair of gold or silver studs, a pair of inexpensive rhinestone studs (either clear like diamonds or one in the color of her birthstone) and 2 pairs of very short danglies and maybe a set of small hoops. Maybe a few hair bands in black, white and pink (or another color that she especially likes) Note: I get mine from the 99 Cent Only store, 3 to a pack like These Stretchy Ones and the plastic ones like These Depending upon her hair style, she might also like some hair clips like These or These and some Hair Scrunchies Maybe a bottle or 2 of some flattering nail polish. I didn’t wear makeup at aged 13, so can’t really advise on that subject. Maybe a pair of black leggings to wear with skirts. And some white and/or black tights if she likes to wear those with dresses and skirts.
If budget is a concern, consider shopping at thrift stores, but go with a plan in mind. Decide ahead of time how many items you need and what type, make sure they are in the fabric/color scheme that you need to mix and match. Make sure your young lady is wearing something like stretchy bicycle shorts and a tank top so that she can try on the clothes over her own clothes, because often there is no dressing room. But you can find bargains and great looking stuff at thrift stores.
The amount of clothing she needs will certainly depend upon the laundry schedule of her parents and the clothing that she gets should be of a good quality fabric that won’t disintegrate after several washings. These clothes should be able to last for an entire year.