Public schools (including charters) are in a bit of a pickle: they must admit all students, regardless of their suitability or level of preparation. Imagine a top medical school stuck with this requirement or a school for rocket scientists burdened with this "come one, come all" requirement.
Does this mean our school must, willy-nilly, become like any other school? On the contrary, there are ways to attract students who would truly benefit from the kind of education we have in mind and dissuade those who are disinterested from enrolling in the first place. As I understand it, the only potential student we could say, "No" to would be a student who has been expelled from a school within the district we are operating in. Here are a couple of ideas for ensuring that the school culture remains positive.
Entrance counseling and placement testing. Given our mission and approach to education, it would behoove us to counsel prospective students on precisely what will be expected of them and their parent(s) in terms of academic rigor, behavior and other things that will help students and parents discern whether or not this is the right school for them. In addition, if we give placement tests, families would be able to see the kinds of academic expectations the school has.
Cap enrollment. Let's not be greedy. This year public schools get somewhere in the neighborhood of $6,200 per student. It's easy to be tempted to say if 100 students is good, 150 is better. The obvious problem is that more students means that the teacher to pupil ratio shifts. One can move very quickly from students sensing a degree of accountability to staff and peers to a sense of anonymity ("I can do what I want; I'm a face in the crowd"). Placing a limit on the number of students we admit is permitted. What also needs to be in place is a method of saying who can be admitted if fair. This usually takes the form of a wait list (first come, first served) or a lottery.
Exit examination(s)? Some research is required here, but might we insist upon some sort of comprehensive exam prior to graduation? It galls me to run into high school graduates who are functionally illiterate. I take it for granted that our courses would prevent this, but it would also give others a kind of objective verification that our graduates are also competent in the "basics." What I don't know is this: is it legal (Can you imagine it being illegal to have an exit for high school graduates? -- I think it completely within the realm of possibility -- absurd though it be!). I recently read that exit exams are now required in 20 states.