The Rock ’n’ Roll Camp for Girls is a sort of riot-grrrl training compound, a modern-day equivalent to those socialist retreats where hippies used to send their children. The intrepid directors tag along for a summer to observe the girls form bands, participate in self-defense classes and shriek, repeatedly, at the top of their lungs. The movie might have benefited from a more concentrated focus and dramatic arc, but it provides a revealing glimpse into young American outsiders. And to judge from the campers’ performances, the most influential band among our nation’s youth is neither the Beatles nor Nirvana. It’s the Shaggs.