They do it under the auspices of the PTA, which is nothing more than a front for the union position. I attended one PTA meeting when my daughter was in high school and found they were all in lock step with the teacher's union. Never attended another meeting....
Some are in lock step, and some aren't.
My school's PTA isn't. The district is closing our school, so our PTA officers are active in trying to split our district and create a new one. The local PTA chapter president doesn't support our position, but we don't care.
However, I am going to have to rethink my position of giving to the PTA next year when we go to our new schools.