That is a lot easier when you can require a transcript from the prospective student, review it, and say, "Uh, no thank you".
There's a private technical college near here that offers EMT and paramedic training. They "guarantee" "100% success in certification and registration" for their students.
How do they get there? They boot students out after they fail (<80%) their second test in the class.
I'm not necessarily opposed to such a policy. It is, however, intellectually dishonest of them to try to tout it as a better school for that reason. Charter schools are free to reject students who will bring their grade averages down.