Calling work done in off-hours "$0" is magical thinking. That teacher costs a lot to maintain. Those of us who are employed full-time don't really have "off-hours", and regardless of the intensity with which we focus in a given 60-minute period.

Sometimes, while at work, I am thinking of leisure. Sometimes, while at leisure, I am thinking of work.

At the end of the day, the school pays their employee. And the rest of society pays them during "off-hours", e.g. in depreciation of common resources if, say, they drive on the road.

My point is that people cost society to keep alive, but some of those people are better equipped for certain tasks than others. The question is only where do those costs come from, and to what particular efforts are the particularly-equipped assigned.