Since you have a bike you are going to convert, use it to figure out what gear ratio you want to use. Ride it around. Set it in a single gear one day and make that ride to school. Does it work? If not, set it higher or lower the next day. When you have a gear that works for you, get down on your knees and count the teeth on the chainring (front) and cog (rear). The ratio between the two is what you are looking for, not necessarily the same number of teeth. For an extreme example a 25 tooth front and 10 tooth rear is going to give the same ratio as a 50 tooth front and 20 tooth rear.

Don't take the brakes off until you know how to ride. At that time you can make an informed decision.