Lowering the saddle to the lowest level and see how it affects your overall position on the bike is a good start point. Coincidentally, a colleague and friend of mine has done exactly the same, and I almost thought it was you until you mentioned about school :p. Some people would stand-and-pedal, but that is not ideal to do all the time!
Another thing to try is that if your saddle slides front-and-back, see what position you require it to be fixed. This is to get your heap ergonomics right. I hope you have got your allen-key turner available at home :)
UPDATE Mind you that frame sizes for road bikes and MTBs vary a bit. What I mean is that the if you want to buy a road bike, the appropriate frame size for you will be slightly bigger than its MTB counterpart (e.g. 19" road/hybrid bike fr == 18/17" MTB fr).
If you are in the UK, the bike shops usually have charted measurements available for you to find out the optimum height and frame size for it. It would really help us if you give further information such as what exactly is the problem that you experience with this current frame size? Bob (or whoever the guy you spoke to) might have said things to sell a bike to you, but he could very well be right about the size. Coming back to the colleague of mine's bike, it was a carrera 18" MTB and his height is 5' 7". It was just about right, but he could have used a smaller frame. But when we adjusted the seat post height, it was alright although we could have used a slightly smaller bike size. If your round trip ride does not contain too much of steep hill climbing, you should be fine pedalling through. Again, it would be good to know what your height is and the size of bike frame you have.
Try lowering your saddle first and meanwhile, let us know your height and frame size. To help you understand, I am 6' 3" and have a perfect fit bike of 19" frame (48 cm). I can probably do with a smaller frame, tried in the past.