Rockville High School was evacuated for about 20 minutes on Monday, the first day of the school year, because of a report of smoke that turned out to be a malfunctioning light fixture, school and fire officials said. 

A tweet from Rockville High School Principal Debra Munk said it all:

"@RHSprincipal Opening day went well-except for an incident with a smoking light fixture that required us to a call 911 & evacuate the building for 20 min."

The building was evacuated a half-hour before the end of the school day.

About 35 firefighters from the Montgomery County Fire and Rescue Service were dispatched to the school at 1:42 p.m. for a report of smoke on the second floor, said Capt. Oscar Garcia, a county fire and rescue service spokesman.

The first fire unit to arrive found no smoke, but noticed an odor, Garcia said.

Firefighters concluded that the source of the odor was a malfunctioning fluorescent light in which the light ballast had overheated, he said.

Students were able to return to class before the end of the school day for dismissal, said Dana Tofig, a county schools spokesman.