Any small cosmetic case will do. This one I saw online at target for $2.99, and it looked perfect for the few things I would need.
Baby lips in any shade you are comfortable with is perfect for a school beauty bag. I would probably go with quenched, just because it gives you the slightest shine, but will go with anything because it is clear. These also double as a chap stick, just in case your lips are dry one day.
Hair ties are a must for your beauty bag because you never know if your current hair tie will break and you need a knew one, or if you decide to put your hair up and you didn't put a hair tie on your wrist that morning. Emi-jay hair ties won't break on you, they double as a cute bracelet, and they won't break your hair like regular ties.
Getting a perfume tester of any easy to wear, everyday scent is great for any beauty bag because with just one spritz, you have an instant refresher. I have been liking my Jo Malone Red Roses perfume a lot lately, and would probably choose to toss this in my bag. I also think Jo Malone testers are pretty convenient because they have an easy to use spray pump, as if it were a regular sized bottle.
Your favorite concealer could be number one on the list of things to put in you beauty bag. It is so helpful if you end up having a skin emergency in the middle of the day at school. You never know when a big red spot is going to pop up on for face, so carrying a stick of concealer (my Body Shop tea tree concealer is a stick, and stick concealers are quite convenient) will certainly be of use.