02/18/1991 at 01:00 AM EST
>This month Nickelodeon introduces two energetic but often vapid series for and about television's most underserviced audience, adolescents. Welcome Freshman (Saturdays, 6:30 P.M. ET) is a jejune sketch comedy, Laugh In for the lunchroom set. Fifteen (Sundays, 1 and 5 P.M. ET) is an overwritten junior-high soap opera with an appealing cast.