Michael Fullan says unless you have a common vision or a common goal, you can't sustain innovation in a school. At heart, very few people go into teaching who don't care about kids. Now sometimes that care gets rusted over, and you have to get out the moral equivalent of steel wool. But if we're in this business to help kids develop and learn, then it just makes sense to work together to figure out how to do that, because we have more power together than we'll ever have individually. Smallness is important, too.
Large institutions are more difficult to manage.
A number of schools around the country have divided themselves into houses or schools within a school. Any institution could do that. Smallness sharpens focus and fosters personal relationships. John Goodlad has said you should not accept any more candidates for teacher education than you can provide a quality experience. He raises that as a moral point.