Merry-Go-Round Enterprises, Joppa, Md., capitalizes on the nightmare of being seen naked in public in a new series of hip, back-to-school commercials airing on MTV: Music Television. One spot shows a student being called to the front of the class to solve a math problem in his birthday suit. Another has a nude woman meeting a geeky blind date in a bar. "It's 11 p.m. Do you know what you're wearing tomorrow" is the tag at the end of each commercial. Ads in Cosmopolitan, Details, Rolling Stone, Seventeen, Spin and Vibe also support the $7 million campaign from Trahan, Burden & Charles, Baltimore.