You can vote for a new school board. Volunteer to help their election campaign. Or run for election yourself. You actually have MORE voice there than with a private school, where losing 1 customer is quite frankly not a big deal.
Well, considering that the last school board election was decided by more than one vote, my voting would have made no difference. Sure, I could run myself, but I'd be running against some academia type who has the backing of the teacher's union, which make up the bulk of the school board voters anyway, because he/she won't make the teachers accountable. (I support vouchers and would never get elected) And even if I could start a campaign to elect school board members who would make schools and teachers accountable, how many years would my child have to go to a sub-standard school while my campaign gains traction, gets the right people elected, and positive changes can be proposed, approved and implemented?
Or, I can send my daughter to the school that I choose today, knowing that she'll have an incredible advantage over all those poor minority kids with parents that can't afford to send their kids to private school.