I would say that there isn't any point for you to get one. First of all, you are basically riding a fixed course every day, so you can measure your exact route a couple of ways, either with google maps, or by asking one of your parents to drive the route and watching the car odometer. Another reason is that high school kids can be real pricks, and there is a very real chance that your bike computer would get stolen or damaged. Another thing is that there is the very real temptation to mess with a cycle computer while you are riding, which is dangerous. Just enjoy the ride instead. Look for something new and different that you haven't noticed before each day. Getting out on a bicycle is one of the few chances people have in this modern world to take a break from technology and the modern pace of life. Enjoy it.
Instead of a cycle computer, you would be much better off spending the money on a set of road tires for your mountain bike, keeping the knobby tires on hand for weekend trail rides and whatnot. With road pattern tires, the ride will be a lot smoother, easier, faster, and more enjoyable.
Cycling is fun and liberating. Don't worry about trying to quantify every aspect of it. You know about how far you go every day, you can measure the exact route in other ways, and as it doesn't change, you can figure out your exact distances with a simple application of mathematics.