I'm sorry, maybe some details are missing in this question. The school would have assigned a bus number to the child and if they have to exchange buses along the route, that number should have been provided too.
Before placing my child on a bus, any bus, I would contact the school, make them aware that your child will be riding a bus and you may have to fill out paperwork to register your child as a bus rider, at that time, they will assign a bus to you and provide the route. At my school, we have to fill out a bus rider information to receive the assigned number or fill out a waiver if your child will be a car rider. So this is definitely a matter that should be handled at the school, don't place your child on a bus until you have made proper arrangements for the safety of your child.