What a predicament!  On the one hand, your child has rights regardless of her "IEP" status and the school administration is obligated to make sure your child (and all others) are safe on the bus.  On the other hand, your child also has a condition that could make her "special and protected too".  It seems to me that the school is not handling this situation at all.  I’m sure you tried everything already.  I would probably take my child off the bus temporarily until things cooled, and continue to pursue a resolution with the school administration.  Putting things in writing can be effective, especially pointing out the negative effect this situation could have on your child.  I would also speak with the school psychologist or social worker to meet with your child as well, for support.  You could also go to the school board for added effect, as well as the PTA/PTO.  Your child deserves to be safe!  Don’t give up!  Good luck!