I do not find this issue a matter for a joke like it is presented here. Any head of an organization sets the rules deemed necessary to run the business effectively, and here it is a school business. I would however put in the contract as a condition, as it is not a simple task to replace a teacher in the middle of the year.

What is happening is that the system is very much abused to take advantage of paid maternity leave, shorter work hours and so on ..

Teachers who are complaining can find another school, I support the principal decision.