That's simply not true. Big city teachers are essentially a cross-section of society. Conservatives, liberals, libertarians, monarchists (okay, just kidding about monarchists).
Now, here's what is true. Big city union leaders are all progressives. And big city school administrators are all progressives. It's that last group in particular that is destroying education.
The public, political, rubber-on-the-road face of public education is in most of urban America is hard left. Our local school district can always find time, people and money for #BLM support, leftist political protest, transgender awareness, or to bash Trump or denigrate suburban conservatives. If there are conservatives or others within their ranks, you would never know it. For all practical purposes, IMHO they don't exist.