Marx on the schoolmaster as a productive worker

 Capitalist production is not merely the production of commodities, it is,

by its very essence, the production of surplus- value. . . . If we may take

an example from outside the sphere of material production, a schoolmaster

is a productive worker when, in addition to belabouring the heads

of his pupils, he works himself into the ground to enrich the owner of the

school. That the latter has laid out his capital in a teaching factory, instead

of a sausage factory, makes no difference to the relation.

—Karl Marx, Capital