Sussex Police said the incident involved a group of teenage girls in Meaden Way about 6pm on Tuesday (September 1).
A spokesperson said: "Officers are currently with the victim – who has suffered a concussion – and her family.
"Anyone with information can report online or call 101, quoting 1062 of 01/09.
"We are aware of images circulating on social media and would ask the public not to speculate or identify juveniles as it could harm our investigation."
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