Crews were called to Julius West Middle School on Great Falls Road in Rockville shortly before noon on Monday, where there were reports of elevated CO levels.
It is believed the issue was a leak in the boiler room.
Students and staff were evacuated while first responders ventilated the building and officials organized the early dismissal.
No injuries or illnesses were reported.
The incident remains under investigation.
Want breaking news in the DMV as it happens, or want to contribute? Join the DMV All Incidents Facebook group.
Click here to follow Daily Voice Olney and receive free news updates.