I'd expect it to be strongly bimodal precisely because it's so self-selecting: You get the up-and-at-'em overachiever parents who can't bear the thought of their children being less-than-challenged at a school full of the kids of those who barely care, and the extremist parents who are so intense about their religion they can't bear the thought of their kids being out in the secular or even not-quite-as-religious world learning about stuff their dogma absolutely denies.

Maybe the children of the extremely religious parents don't make it into the studies.