FORT MONTGOMERY, N.Y. (CBSNewYork/AP) — A chemical mishap in a high school science lab Tuesday sent more than two dozen people to hospitals to be checked out, and forced the evacuation of the school for the rest of the day.

The Highland Falls-Fort Montgomery School District announced on its Web site Tuesday that around 9:40 a.m. at O’Neill High School in Fort Montgomery, 27 grams of ammonium chloride were placed in distilled water, and allowed to dissolve in the fume hood on a hot plate during a teacher-led demonstration.

The hot plate was supposed to be turned off, but it was inadvertently left on, and the water and chemicals evaporated into a “billowing fog,” the school district said.

The local fire department responded to the scene, and exhaust fans were placed in the lab to clear the area. Everyone was evacuated from the school.

Fire crews checked on students, faculty and staff, and 16 students and one adult were treated and released at area hospitals.

School was dismissed for the day and Hazmat teams were dispatched, and an outside contractor conducted a final cleanup by 3 p.m., the district said.

The school will be back open Wednesday.

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