Koché’s excellent pre-fall show puts an exclamation point on the year’s most enduring trend.

Yesterday at the Strand bookstore in Manhattan, several dozen fashion editors, buyers, and Bushwick-famous stylish kids gathered in the rare-books room and huddled in the narrow passageways between the stacks. As bemused NYU students looked on, the music came up and a legion of street-cast models emerged for a stroll through the store in stretchy suits, wool warm-ups, and camp shirts made of chopped-up jerseys. Several senior citizens, unfazed by the disturbance, wandered onto the runway and caused a momentary model traffic jam...