Also, you would not be "shutting the door" on ever working in a school setting if you completed an internship in a clinical/CMHC sort of setting. NASP accredition is not, to my knowledge, required for a job in any state. Rather, NASP accredition is a certification demonstrating that you have the background consistent with what the organization (NASP) recognizes as requisite training to work as a school psych. Its not required in any state to have the NASP credential. State boards of education credential individuals to practice school psychology in the schools. The NASP credential is most helpful for 3 tangible reasons that I can think of:
(1)Could be helpful for getting a job (especially if you a applying for positons as a MA/CAGS applicant),
(2)Could result in a slightly higher salary (many states/school districts will offer a bit more (1 or 2 thousand annually is what I have seen/heard generally, though it can vary from $0 more to perhaps a few thousand).
(3)Transportability issues. That is, a number of states (though only a minority of total states, I'm not sure of the total number) recognize NASP credential for state licensure purposes. So if you move in-between states, having that NASP credential can reduce the bureaucratic hastles involved with getting licensed state DOE.
Main point - you can still work in schools without 600 hours of school-based internship. You could, theoretically, complete an APPIC internship in a non-school setting, then get a job in a school (without NASP certification) and then, after a year, apply with those hours completed working in the school, for NCSP credential. Of course, you should check with your state board to make sure what I said is correct regarding internship requirements. I do know more than 1 individual that went the route I outlined.