The evacuation began some time after 9:45 a.m. at the William Davies Middle School in Mays Landing.
School officials say a strong odor was detected in the area of the science classrooms, and administrators were concerned that it might be from a natural gas leak.
The evacuated students were put on school buses and taken to the George Hess Educational Complex Elementary School on Babcock Road in Mays Landing.
School officials say the students were given food and finished out the school day there.
Officials determined that the smell was coming from a sewer drain. They're still trying to pinpoint exactly what may be causing the odor.
No injuries were reported among students, faculty or staff.