National City Middle School was evacuated Monday afternoon after a fire sparked in a portable classroom's bathroom on campus.
The fire broke out at about 3:30 p.m., according to Sweetwater School District spokesman Manny Rubio.
The portable units have not been used for about three weeks, since the school held a ribbon cutting ceremony for new, permanent classrooms at the school.
Construction crews were in the process of tearing them down when the fire started, Rubio said.
Officials believe the flames may have been started by a student or were "student-related," but it is still being investigated.
All students and staff doing after-school activities escaped unharmed. Rubio said the evacuation was just a precaution.