I tried to be too clever with that link, so it will come up saying that there's information missing.
Use the GB advanced search for the start and end of the journey
On the assumption that you don't live too far away from school, walk 100 yards at your average walking pace, and see how long it takes you. Then walk to your school at the same pace, and work out how far it is. The advantage of this method, is that it gives you the distance in real, rather than "as the crow flies" terms, and also it provides good exercise.