My daughter is 10 and we just finished doing our first back to school shopping. JC Penny has some great jean capri's on sale for $9.99, they also had some cute jean leggings for the Fall that would be versatile. I would keep it to dark to medium colored denim or black. I agree with everyone else. T-shirts with cute sayings are a big hit. They also had some fun striped shirts at JC Penny you could wear with jeans and would be comfy for school. Is it cold where you live in the Fall? A zip up hooded jacket out of sweatshirt material would be nice to layer on top. My kids wear them to the bus stop and we live in Florida.
Target usually has underwear and socks on sale this time of year for their big multi-packs. That would be a good addition because who doesn't need socks and underwear? Old Navy graphic t's are always a big hit here. You can get the zip up hoodies there as well.
For school supplies do you know what school? Our schools have a certain set of supplies the kids have to bring. If you don't have a list then things that are always good to have at home are extra pencils, spiral notebooks, glue sticks for projects, thin and thick markets, erasers. Target always has a few things on sale super cheap each week.
As for backpacks, I'd go simple with no characters because at this age personalities are so different. Turquoise, black, pink are all big right now and mix of those would cover all bases in my opinion.
It's so kind of you and your co-workers to do this! I'm sure this little girl will be thrilled.