Here's my hunch.

1. The teachers were well compensated for what was to them was an easy short bit of work, with the rest of the summer off.
2. There was a minimum number of students needed, or the trip would be cancelled.
3. The teachers did whatever it took to get to the number of students needed for the trip, and their extra pay, to proceed.  They ignored the requirements, pressured parents, etc., whatever it took.

It's just a hunch, but...