Well, it’s pretty easy to dump on the school for this, but did the dad submit an alternative lesson plan for the trip? Most schools would have allowed that.
A trip to Boston for the Marathon could qualify for credits in:
Current events
Physical education
Studies about terrorism
Law and order
Also, if tours of local monuments and landmarks are included:
History
Political science
In my day kids would have done their math homework while absent and turned it in when they got back, and written an essay about their experience during the trip
and the sun would still rise in the east and set in the west
principal sounds like “she who must be obeyed”