They do so as individuals and school employees not as union members, they do have to follow curriculum as well and I do agree that they should leave their biased political views at the door of the school, this applies to consrvative teachers as well. Parents should raise kids, not teachers.
I'm imagining some secret cabal of the wealthy and powerful, making a blunder in their manipulations of society: deciding that US schoolteachers, who naturally will be a formative influence on children, should be educated and underpaid.
Within 2-3 generations of that nurturing of awareness and identification with lower socioeconomic classes, the entire populace ends up inclined to look out for equity for each other, and tears down the cabal.
