The images were posted to the Harriton High School event attended by nearly 300 students, and shut down as soon as counselors were notified, the Lower Merion District said.
The district announced a switch to full-remote learning for its high schools just this week, following a petition launched by students.
District officials believe the event was set up without usual security measures.
"This kind of 'Zoom Bombing' is completely unacceptable and if the source of these images can be identified, they will face serious consequences, especially if it turns out they are a member of the LMSD community," the Lower Merion officials said.
"LMSD is a District that prides itself on promoting inclusivity and belonging for all of its students and staff. Depictions of racism or sexual exploitation have no place in our schools or community."
The incident remains under investigation by authorities.
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