Never mind, it was just a branch breaking

Catskill High School and Middle School went on lockdown on Thursday after the superintendent heard what she thought was a gunshot. Turns out it was actually a tree limb crashing, she later told the Daily Freeman:

The schools immediately went into lock-out mode, meaning that no additional people were allowed inside the buildings. School officials then contacted village police, who scouted the area and set up a perimeter. “They advised us that there was tree removal work being done in a place very close to the area. They believed when the tree was felled, it gave off the sound of a gunshot,” Farrell said.