I work for a Primary school as a teaching assistant. My paid hours have been cut repeatedly over the last couple of years and I've made up the financial shortfall with more live band work and teaching guitar and bass. However when I get asked if I can do some extra hours at school(say for covering a colleague off sick), if I say 'sorry, got a band commitment' they get a bit snotty.
It's my experience in a very varied 35 year working career that employers try to have it both ways - if you let them get away with it.
But, as I advise youngsters - If you act like a carpet, people will walk on you