Sorry that your principal sounds like she's being somewhat of a jerk. Our school offers bussing for all students, too, and the drop-off lane can get pretty crazy in the morning.
Our school has a "no walkers" policy, too, but I've walked my child to school and nobody's ever complained. I think what our policy means is that they will not release a child from school, except to his assigned bus, without a parent signature. So if I want my child to walk home from school, I need to walk to school, sign him out, and then walk home with him. They wouldn't let him walk home by himself, even if we lived next door. Once I've signed my kid out, though, I don't think they care how we get home -- car, bike, on foot. I don't have a problem with that policy because it's for safety.
Good luck at finding something that works for your family, and your school.