A hot-headed teen got a slap on the wrist Friday for slugging a Manhattan principal who told him to turn down his music. Judge Edwina Richardson-Mendelson granted Luis Penzo, 19, youthful offender status and sentenced him to a conditional discharge — as long as he stays out of trouble for three years he’ll dodge prison and a criminal record. “You made us very proud,” the judge said of the surly teen’s compliance with a family therapy program. Penzo, who sauntered into Manhattan Supreme Court Friday wearing a white T-shirt and red gym shorts, offered no apology.