Officers responded to Richmond High School at 1250 23rd St. just before midnight, about an hour after the dance ended, and saw several males running away, Richmond police Sgt. Bisa French said.
The officers then found the victim unconscious and suffering from "different injuries indicating she had been assaulted," French said.
She was airlifted to an area hospital in critical condition, and has since been listed in stable condition.
Police believe the girl, who is a student at Richmond High School, may have been sexually assaulted as well, French said.
The officers detained a 19-year-old man who was seen running from the high school shortly after and arrested him on suspicion of rape, French said.
Authorities believe there are more suspects. Police believe the victim knew at least one of her attackers..