A female school safety agent was stabbed in the leg as she tried to break up a fight between two middle schoolers in Brooklyn Wednesday, cops said. The 53-year-old agent stepped in around 3:10 p.m. as the youngsters brawled on Beverley Road near Ralph Avenue, steps from IS 285, the Meyer Levin Middle School for the Performing Arts in East Flatbush, police said. The agent was knifed on the calf and taken to the Kings County Hospital Center, where she is listed in stable condition, authorities