Pottsgrove Middle School was evacuated early Thursday after close to two dozen students became sick while rehearsing for a holiday pageant.
Three students fainted and the rest reported symptoms of nausea, headaches and dizziness.
Police tell NBC10 that it was not carbon monoxide-related.
School officials say there is no medical reason to explain the symptoms. They believe the three students passed out because of the hot lights on stage and the rest probably felt anxious after watching their classmates faint.
All 21 students were taken to Pottstown Hospital, where they were treated and released.
The pageant, which was scheduled for Thursday night, was postponed.
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