Dottie- you are correct that charter schools split the traditional pool of money for public schools. However, in my district, the school district is about to be taken over by the state, due to extreme mismanagement of funds, corruption by politicians and board members who are trying to get elected to higher office but know nothing about education. The charters are doing just fine- because they have complete local control over their funds. In a year, the charter schools will be the only ones left standing.
In many cases, you cannot assume that the ADA of the public school would go up if the charter did not exist. Many parents would still chose other options for their students, particularly in very low performing districts. There's no way my son would attend the regular public school. The district GATE people have repeatedly told us that they have nothing for him and cannot accommodate him in a regular class. They've basically pulled the reverse of some charters and special ed students- "you can send him but we won't teach him." Too bad for them- they could use his standardized test scores :-)
In addition, most charters in CA do not have teacher's unions. While there are certain protections teachers' unions provide, most of my teachers are delighted that they work with colleagues who are well-trained, well-paid and if someone is slacking they get an improvement plan. If they don't improve, they are at-will and leave. It's that simple. Our district is currently paying 40 teachers to not teach because they have too much seniority to fire.
In an ideal world, with schools run by well trained educators, not politicians, I would love for every school to be run like a well-oiled machine. Students needs are met, teachers have additional training beyond the one year credential program and parents are welcomed into the classroom as advocates, mentors and volunteers.