I have had schools go weird on me that my child needs to do x or y because of his socialization. Could it be something along those lines. They think he NEEDS the experience going on the bus, following directions, etc..

Another possibility for their odd behavior. Are they asking for a donation for the trip? Field trips are one of the few things schools in my area can still ask for $$ to help defray the cost. Yet, they don't want to deny any child the ability to go on the field trip. So if I child a parent didn't return the money & permission slip the school might assume it was because a parent couldn't pay and didn't want to say.