The young victim suffered fatal head and stomach injuries during an altercation with "several female students" in the girls' bathroom of Howard High School of Technology in Delaware on Thursday morning.
"She 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," one student who witnessed the deadly fight told local media.
She was taken to hospital where she later died with her parents at her side.
Students said the fight broke out after an argument about a boy.
Police said they were interviewing witnesses but did not confirm whether any arrests had been made.
