I feel like this would be a strong argument against homeschooling (and private religious schools for that matter). Because if they're home-schooled then they'll get exposed to fewer people from different social backgrounds than themselves, and that would reasonably reduce the amount of "brainwashing".
It depends for sure. However most households have a diversity of friends and the kids do have to go in group outings. Besides, depending on school district your claim might not be accurate (ie you may attend a school full of rich brats, you may attend a school full of ne’er do wells, or a school where everyone is on the lunch program) neither broadens horizons).
Religion might be a problem with strictly religious parents, but I don’t see the damage being worse than that done by school systems. I experienced way too many friends who didn’t fit in and dropped out and other kids who believed in the system and believed things their teachers told them.
