No one was injured, they said. A lockdown at the school, Achievement First Amistad High School at 580 Dixwell Ave., has been lifted.
One round of gunfire tore through a rear window of the building about 8:10 a.m., Capt. Anthony Duff said shortly before 10:30 a.m.
Anyone with information about the gunfire is asked to contact the New Haven Police Department at 203-946-6304.