For a primary school trip, it should have been organised to take all but maybe they didn't think as many would actually want to go.
However, 'first come, first served is not fair either because kids who come from poorer homes might need longer to get the money together. When I started teaching, we always did names out of a hat when a trip was over-subscribed. I can't actually think of a fairer way of doing it. (The kids didn't see us do it; they were just told who was going and the rest got a refund.) In the end, we could only take a certain number on a coach and there weren't enough to warrant a second coach, so it would have massively bumped the price up.
The issue with going to Disneyland is separate. Again the school I worked at restarted a year 8 trip to France and the day in Disneyland was a 'carrot' to encourage kids to go (school in deprived area of North England) as attitudes to were quite negative; and it worked - trip was full. The next step was to add a French theme park with the intention of dropping Disneyland once the trip had a positive vibe about it and kids wanted to go.
Prepperpig, there is a massive advantage to the school in another adult volunteering to go - it means they don't have to pay for supply to cover a member of staff. For a while it used to be 2 teachers, the Head and a member of the admin team. Then a teacher brought their partner (who was a retired teacher.) It makes the trip more affordable for the kids as supply has to be included in the costs.