If all you plan on doing is riding to school and riding on the road, get a hard tail bike from Walmart and have fun. I'll repeat this, as it needs to be understood, do not buy a full suspension bike.
Find something from Walmart or Sport Chek that's in your price range and fits you, then take it to a local bike shop to be set up properly. If you go this route, do not take it off road. It may be fine the first time, hell it may be fine the first ten times. But a cheap bike will end up breaking, possibly hurting you, if you use it off road.