It happened in the cafeteria of Pennbrook Middle School in North Wales, Montgomery County on Wednesday.
According to Upper Gwynedd police, surveillance video shows a 7th grader walking along a lunch table when the accused student runs towards her and begins hitting her in the head with a Stanley cup.
Officers described the incident as an "unprovoked, sudden, violent attack." They say the assailant came from behind and immediately began hitting the student.
The accused student will likely face several charges in this incident, including aggravated assault, investigators say.
WATCH | Montgomery County parents outraged after middle school student attacked with Stanley cup
Montgomery County parents outraged after middle school student attacked with Stanley cup
On Thursday, more than 50 people packed a North Penn School District meeting to voice concerns about the attack.
"This was avoidable and the district truly failed to protect the students at Pennbrook," asked one parent at the meeting. "What are we supposed to say to our children after this? How do I send him back to school?"
The school district did not respond to any of the parents' questions at the meeting. Officials say the incident is still under investigation.