Just meant to add , You are living in school property on school premises in order to be available for your job,you do not have a tenancy so what the employer says goes.
Sorry, but I do not see you have much chance here, the school will say that you need permission to even have an overnight visitor.
I sympathise (father of 5 myself) but you have to realise that once a child is 18 they are legally not your responcibility.
From a purely legal standpoint there is no diffrence between having an adult non dependant son live with you and having a lodger.
It will come down to the schools insurance , which will not cover having adult non employees living on school premises.
That has just given me a thought, it it at all possible your son could get a job working with the school ? maintenance or something? that may solve the problem.