The incident happened at ABG Schultz Middle School in Hempstead around 11 a.m., while students were registering for summer classes.
Several parents and students were in a computer lab when police say 15-year-old Oumar Barry ran into the lab and threw a lit firecracker onto a table that held a computer. That computer exploded, sending shrapnel across the room.
Five people were injured and taken to a local hospital. One individual suffered burns to the face and arm, while two others were treated for leg fractures and another for pain to the knee.
Also among the aided was a pregnant female who was treated for stomach pain. All the injuries are considered non life threatening.
The mortar exploded three different times causing damage to the floor, walls and ceiling.
The incident set off a panic in the hall of the school.
"I heard, like, three pows," one student said. "It was like boom, boom, boom, like gunshots going off inside the building. And I seen a whole bunch of people running and screaming, it's a fire, it's a fire."
"A straight boom, like basically a bomb," another said. "You know, when you throw a firecracker indoors, it's going to sound loud. So it was really loud, a lot of smoke and everything. The teachers were panicked. Some of them were crying and everything."
Police say this is far from an everyday prank.
"We're investigating it and we're gonna charge the highest count that we can. Any time somebody throws an explosive device into a crowded room, we have to take that seriously," Chief Joseph Wing said.
Barry is being charged as an adult for 1st Degree Arson. He's due in court on Wednesday.