If you are riding to school - I would recommend something used. Shop craigslist / local classifieds for a used bike. I picked up a used hardtail mtn bike for $25. No one would give it a second look on a school bikerack. Look for something that can take a bike rack and fenders. A 3.5-5 mile commute is easily doable on mtn bike or hybrid.

If you will be sharing the road with cars doing 50mph, make sure you have good lighting for evenings. I would wait to buy anything new. Save your money until you have a clearer idea of what you want - what your personal riding style is.

Good luck.