Private school is having a fashion moment. Buttoned blazers, knee socks, crested sweaters, university-striped scarves, rep ties -- all those stodgy restrictions of self-expression are looking like high style this fall. From the Balenciaga runway to the pages of theJ. Crew catalog, designers are turning the preppy uniform into something actually cool and wearable. Why do we feel so ready for it, after a summer of free-flowing style? Maybe it's the Harry Potter effect. Call it "Hogwarts Chic."