About 1,500 students were marched to nearby P.S. 48, where they waited in a playground as their building was ventilated.
The elevated levels of carbon monoxide were detected by EMTs in the school on an unrelated call.
No injuries were reported.
The four schools in the building are P.S. 168, the Hyde Leadership Charter School, the Vida Bogart School for All Children and the Hunts Point School.
Students were sent home for the rest of the day.