MORRISSANIA - A Bronx high school student is headed back to class after his gym teacher rescued him from a heart attack earlier this month. Louis Flores, 17, a junior at the Bronx Center for Science & Mathematics, collapsed in gym class on Dec. 8. Gym teacher Steve Greenwald says he rushed to get a defibrillator when he noticed Flores wasn't breathing. Greenwald, a 26-year veteran of the city's public school system, says his training immediately kicked in. Doctors informed the teen's parents that he has a heart defect, and he underwent successful surgery to correct it.