You must not work for free. It is bad enough with people working for airlines for free, or even paying to do so, without instructing becoming another victim. Schools charge customers for your services, so you should get paid.
If you want to fly as a hobby then do so, but do not undermine things for everyone else!
Lots of schools want part time instructors so don't even think of offering to work for nothing. If you get asked to then tell them to take a hike.