The discovery was made at the New Foundations Charter High School in Northeast Philadelphia.
In a letter obtained by Action News, parents were notified on Monday about the shooting investigation.
"A bullet hole was discovered in one of the windows of a third floor classroom on the Tulip Street side of the building. The bullet was still in the classroom, and police were immediately called," read a portion of the letter.
The incident happened sometime over the weekend.
No injuries were reported.
Philadelphia police say the investigation is active and ongoing.
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