The 16-year-old, named by classmates as Amy Joyner, died following a fight at a Delaware high school.
It’s believed several students were involved as pupils arrived for the start of the school day.
Police are looking into whether students recorded the attack on their mobile phones.
One student says she was in a cubicle in the girl’s toilets when a fight broke out.
Kayla Wilson told Philadelphia TV station WPVI that the victim “was fighting a girl, and then that’s when all these other girls started banking her, like jumping her, and she hit her head on the sink.”
Two female students were taken to police headquarters for questioning.
“I’m so upset that the young lady lost her life today. Things like this shouldn’t happen,” Mayor Dennis Williams said.
“My heart bleeds for the family.”
A vigil for Amy was held on Thursday night.
Shortly after the attack #RIPAmy started trending on Twitter.
Police are awaiting the results of a post mortem to determine her exact cause of death.