The public schools (at least in Washington State) are run by the teachers' union, which is the most powerful political entity in the state.
Typically the state legislatures dictate what is in the curriculum broadly, sometimes extra specifically, and the administrators and teachers create their lessons to satisfy the legislation.
How does the teacher's union suppress subject matters in the history department?
It's very hard to get elected in this state without a teachers' union endorsement.
It's also hard to get elected without people voting for you, or funding for political campaigns.