Just because you can't do it, does it mean that other people can't do it?
I say if their school admin doesn't mind, they should go for it.
I think that when your business has a cycle, you have an obligation to work that cycle. Teachers have a definite cycle. It isn't like you don't realize what their business cycle is moving in. Retail has a cycle. Tax accountants have a cycle. These are jobs that you have a responsibility to do your job when the work is there - but there are other times of year when the work isn't.
That isn't all jobs....a lot of jobs aren't cyclic.
If the school admin doesn't mind, that's nice for them. The school has other stakeholders. The students, the parents and the taxpayers.