Firefighters converged on the school in the 3600 block of Filbert Street at 2:00 p.m. Friday after reports of fumes in the building.
A fire department spokesman tells Action News it turned out that 8 ounces of Potassium Dichromate, a toxic oxidizing agent, had spilled in a chemistry supply closet in a third floor classroom.
The spill happened a few days ago, but was reported for the first time on Friday.
The third floor was evacuated, but all students remained inside the building.
A hazardous materials team on the scene cleaned up the spill.
The entire incident was declared under control within half an hour.
No one was sickened.
School officials said classes were being conducted normally and students would be dismissed at their normal times.