You can never be over-dressed but you can easily be under. I would recommend some slim slacks, navy, with a nice colored blouse and matching 3/4 sleeve blazer. A peachy color would look nice underneath, white always looks classy though. The do sell placers marketed to Juniors which are often adorable and well fitted but still give off a sense of professionalism. Simple jewelry, studs or short dangles are a nice compliment to the outfit. Short heels or flats, but I don't recommend high stick heels unless you are sure you can walk in them.
Casual business ONLY!
For your age and the fact that school is something I would say something like a simple formal dress, not many accessories, probably trying to look serious about your outfit, nothing flashy, with this will create a good impression
A tank top that is not too low cut, with a knee-length skirt, and a blazer would look very professional and pretty, while keeping you cool.