This is an example of Union solidarity.
Explanation:
This is one of the examples of why having a worker's union for your profession is a good thing for ensuring your own rights as a working professional.
If there was no Union then the school could just have made the people work and they would have had had to work because they would not have any other choice.
This is not true when there are unions in place to keep a check.
The union here has put enough pressure to call an arbitrator.