They're mocking the fact your running, and therefore likening you to Forrest Gump.
A thought to present though: A least some other people get to class on time, and they don't run to class do they? Therefore it must be concluded its possible to reach class without running. If you were to not run, potentially people wouldn't mock you in the interval between classes? Additionally, it is undoubtedly crowded in the corridors of your school between classes, if that is the case, running isn't likely to save you much time. Carefully watching which way people in front of you are moving, and slipping through them would save you more time, and if you learn where more people bunch up in the corridors, you might be able to work out a more efficient route. Thats just my opinion from experience of ~5 years of Secondary School and the first year of Sixth Form.
Alternatively, just continue sprinting to classes, I would present one word of warning however. If there are particularly malicious people at your school, they may advance to attempting to trip you up, which is hard to avoid when running in a crowded corridor, if your walking, (even if it happens to be a slightly weird walk, like mine is), you have more control over your movement, and then rather than putting speed into your movement, you can simply increase the force of your movement. They'll either wisely move their foot out the way, or get dragged a few feet and stumble a bit, then they shouldn't be as much of a bother.