Another "drill" lockdown. Note that the student population "erves 300 students with special needs, including those with severe emotional disabilities, autism, cerebral palsy and other disorders." This is a targeted population for these kinds of scenarios. Lockdown Drill Scares School in East Harlem - NYTimes.com
Sometimes the local police are in on these drills, but in this case, they weren't. They arrived at the school in response to a 911 call made from within the school.