A 20-year-old was arrested in from of the John Jay Educational Campus on Seventh Avenue on Wednesday. View Full Caption DNAInfo/Amy Zimmer

PARK SLOPE — A 20-year-old was arrested Wednesday for allegedly stabbing a teen in the back and hand, biting another teen in the chest and slashing the ear of a man in front of the John Jay educational campus while a group of junior high student bystanders were caught in the melee, according to police and witnesses.

The 20-year-old’s charges were pending in the incident that took place around 11:30 a.m. in front of 237 Seventh Ave., just as the last day of school was wrapping up.

The 19-year-old, who was stabbed, was in stable condition at Methodist Hospital along with the 17-year-old who was bit and the 42-year-old man slashed on his left ear, police officials said.

It was unclear whether any of the men were students or staffers at any of the four schools within the John Jay campus, where families in recent years have been calling for the removal of the building’s metal detectors.

Marc Williams, principal of the High School for Journalism said, “We are still investigating the details of incident,” declining to comment further.

The incident occurred just as a group of sixth graders from Park Slope Collegiate — one of the other schools in the building — was returning from a trip to Prospect Park.

They were saying farewell on the sidewalk at the corner of Seventh Avenue and Fourth Street, as their teacher handed out report cards, when the fight broke out, the students recounted.

“The guy pulled out a knife and the teacher told us to run so we all started running,” said one sixth grade girl.

She and three friends ran down the block into a parking lot behind the school.

“I started crying,” she said. “And the guys came in the parking lot. So we ran inside the yard into our school. I was hiding behind a tree. I was hyperventilating.”

Her friend said that when she sprinted into the parking lot, she soon realized she was running next to the alleged stabber.

“I didn’t realize the guy was next to me,” the friend said, “and I tripped and I got so scared – and the knife was just like passing me by,” she said, pointing an inch away from her face.

The sixth graders did not know what the fight was about, they said, and noted they didn’t think it involved anyone from their particular school, which is sixth through 12th grade.

Staff who answered the phones from the other two schools, Millennium High School and the Secondary School for Law said they knew nothing about the incident.

“Safety always comes first,” said Department of Education spokeswman Toya Holness. “The NYPD immediately responded to a serious incident that took place near the John Jay campus in Brooklyn. Students, staff and families are safe, and individuals involved are being treated for minor injuries.”