More like parent lack of control. Who else is on the school boards and PTA's. Unions have a lot of control, but parents have the ultimate control.
Parents are too busy to be involved in their child's school, thus giving away control to those activist they complain about.
Everyone complains about the state of public education, as they should, but few are proactive enough to force a change. It is easier to sit back and use teachers as a whipping post then to actually fight for change.
This is not the case where I live. The school board members are often family members of NEA members and parents hesitate to complain because of fear of retailiation against their child. Teachers have the power to grade, after all, and many grades are subjective (even math)