It happened around 15 minutes after dismissal at I.S. 68.
Police say the 6th grade male student, 12, reportedly took a lighter from his pocket and accidentally set the hair extensions of the female student, an 8th grader, on fire.
The victim was taken to Staten Island University Hospital's burn unit.
The boy is not expected to be charged. Police believe the boy was playing with the lighter and got too close to the girl.
One student captured the incident on his cell phone.
On the video, students are heard screaming about their classmate, shouting "Her hair is on fire!"
Officers were by her side right away, one of them with a fire extinguisher.
The 8th grader is seen leaning down as she tries to recover, but she is weeping, and moments later she was taken to the hospital.
Initially, authorities believed the incident resulted from a verbal altercation, but now they say it was the result of horsing around.
Police say the was playing with a lighter, teasing the girl by flicking it near her face.
"She was screaming in pain, and they were trying to put the fire out," one student said. "It's sad, because she's a nice girl. She's very sweet and kind to people, and for this to happen to her, it's sad."