I would let the kid decide but make clear there would be no screen time if she decided to stay home
WTF!!! I just don't get this, I see so many parents who let their 7-8-9-10 year old children decide EVERYTHING. The kids literally run the households.
I totally get that you aren't going to force them your child to take ballet, but it's a field trip and what her school has decided is appropriate for an educational experience.
You teaching your daughter that if it's discomforting that it's okay to back out, to not challenge yourself. Maybe the last field trip was a fiasco, maybe it wasn't.. every child's needs are not going to be met on every occasion and that's okay too.
You did right by calling the principal but you really want to focus on the chaperone on the bus aspect, no one would put kids on a bus without a bathroom.