It happened in a second floor classroom at the George F. Baker school in the Tuxedo Union Free School District.
Police say a teacher heard noises and called a janitor, who noticed that the ceiling was buckling.
The teacher and students rushed out of the room just before the ceiling came down.
No one was injured, but all staff and students were evacuated to the school gym.
The school is one of two schools in the Orange County district.
The school dismissed all students at 10:15 a.m., as gas was turned off to make the repairs. There is no word yet on when the repairs will be completed.
The building was constructed in 1931.