Our school committee has come out publicly against allowing our kids to go to charter schools because our public (town) schools are rated so well on the state tests. They want it to be a state law. Of course, these tests only indicate the percentage of kids who get over a minimal proficiency cutoff point. They claim that this means they provide a quality education, so kids should not be allowed to go anywhere else.
They see it as their money that they are losing. It's not an argument of marginal costs. They have experience with laying off teachers. It may not be a continuous function, but their argument is more fundamental than that. It's about control. Charter schools challenge that control.
When our son was at a private school (grades 2-5), one parent seemed quite satisfied that it wasn't a charter school that siphoned money away from her daughter's school. However, charter school money is not part of our town's school budget calculations. It's a separate budget category. School funding is primarily driven by the number of kids per class.
We got a certain amount of negative (elitist) reaction when our son was in a private school in the early grades. Now that he is back in the public schools and is headed to high school next year, everybody asks where he is going (because he is a top student). Apparently, it's elitist in K-8, but quite understandable for high school. When we tell them that he is going to the public high school, we get a lot of happy reactions. Perhaps it's a vote of confidence for where they are sending their kids.