The stabbing took place at the school which had been rented out for a "baptismal event" by a private party, said Haverstraw Police Lt. Martin Lund.
During the party, a large fight broke out that involved numerous people and spilled out onto the street, Lund said.
The unidentified man was stabbed on the street, he added.
Three 16-year-old girls who reportedly started the melee were arrested and charged with felony assault, Lund said.
They are being held on $15,000 bond at the Rockland County Jail.
Lund said, no arrests have been made in the stabbing, but the investigation into the incident was continuing.
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