Not everyone lives in areas served by a bus system... none of the paperwork I have ever seen has spelled out any of this like that... it may depend on the school you are attending. I don't know, I haven't found the law or any official statement by the Dept of Ed. on it.
My point is I'm not sure it is actually a rule- I can't find anything official that says that it is a rule. If it is a rule, it is a stupid one- but that is indeed a different discussion.