Though the town’s key twentysomethings have converged on Riverdale High as a new locus of action, “Back To School” nonetheless takes on the fragmented feeling of the downbeat episodes that fill out the midsection of each season. We’ve got some sturdy narrative potential from the recruitment of the hottest teaching faculty in human history, guaranteed to yield classfuls of A’s in at least economics and shopwork, and Archie’s plotline works comfortably within that sphere. But we’ve officially reached Lots Of Stuff Going On territory, with Cheryl Blossom forging paintings and Jughead on what seems to be a wholly tangential adventure and Betty tracking down her sister, who’s not so dead after all. It’s an eventful hour, and in spite of a an instantly immortal karaoke performance, that’s all it is—full.