School is asking for volunteers for a school trip. There is no mention of being paid for those hours, and no pressure put on people to volunteer.
I'm a part-time TA, and I work on that day, so of course I have to go on the trip. But the trip is on one of my part-time days, and will return at least 1h after I would normally finish. I also have previously arranged plans for that afternoon.
What do I do?