My school district did that this year. The principal told the bus NOT to pick Troy up unless they heard from HER. (when he had his sprained ankle) I told her directly, as a matter of fact, "That is a violation of Troy's rights. You are discriminating against him. The only reason that an administrator can call off the bus service is if a child is suspended". I don't know if your WA laws are the same, but I would imagine it is.
Here is a link called, "Is It Legal to Send Disabled Kids Home Early?"
http://www.wrightslaw.com/advoc/ltrs/kids_sent_home.htm
