White shirts, blazers with insignia, striped ties and little pleated skirts — school uniforms have a charm that has made them fashion perennials. But for fall, designers were doing intriguing takeoffs on them, taking what’s usually crisp, starched and seemingly immutable into new territory. There were natty, double-breasted plaid raincoats, flared leather skirts, long shorts striped like a school jacket and even a jumper reminiscent of an old-fashioned gym uniform.