A Bronx student stabbed a classmate to death with a pair of scissors and seriously injured another inside their high school this morning. Investigators believe an 18-year-old plunged the tool into a 15- and 16-year-old around 10:50 a.m. on campus. The motive was not immediately clear, but investigators believe the attacker may have been the target of repeated bullying, the sources said. The 15-year-old, who was stabbed in the chest, was pronounced dead on arrival at St. Barnabas Hospital and the 16-year-old is fighting for his life. The 18-year-old has been arrested.