Private schools can expel (or refuse to admit) underperforming or problematic students. But in most states, charter schools have to follow the same admissions and disciplinary policies as any other public school. That includes taking special education kids. They don't get to pick and choose.

Given the large difference in cost between average students and expensive students (ESL, special needs, behavioral issues, ADHD) I expect that charter schools will find creative ways to ensure they minimize the latter. (Similar to how health insurance companies compete for young, healthy participants.)