They are allowed to screen prospective students up front. They also won't kick out under-performers for getting Ds. They will find a disciplinary reason to do so.

Every one of us could have been kicked out of school at one time or another if we had fallen under the microscope looking for an excuse.

No, that's also misinformation. Public charter schools in most states aren't allowed to screen prospective students up front. Any parents can enroll their children, and when a charter school is oversubscribed they use admission lotteries. And they follow the same disciplinary procedures as other public schools.