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 in a statement that 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.
In the statement, the school district tried to explain further what might have caused so many kids to become sick:
School will resume as regularly scheduled tomorrow.
It has been reported from medical personnel that there is no physical cause for the vast majority of the students who were transported based on their reported symptoms. We believe that students who witnessed the first three children faint became anxious and panic ensued. We ask that you reassure your children this evening that this was an isolated incident and that they are safe when they return to the middle school tomorrow.