BENTLEYVILLE, Pa. —

Middle school students and staff at one Western Pennsylvania district were evacuated Monday after a teacher noticed a haze inside the school's gymnasium.

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In a letter posted on Facebook, the Bentworth School District said its middle school experienced an "unexpected situation that required an immediate response," prompting the school building to be evacuated.

The letter said a staff member noticed a haze in the school's gymnasium as students were being dropped off. Out of concern and caution, the school activated the fire alarm and evacuated the building.

Buses were brought in so students on site had a warm place to wait. Students who were in the process of being picked up were taken to t

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