A 16-year-old boy needed hospital treatment after being found with a head injury inside the Tesco Alliance on Broad Street yesterday evening (27 July).
He was discharged earlier today.
Police say a group of youngsters were involved in 'an altercation' inside the store at 7:25pm.
Five people have been arrested and bailed pending further enquiries.
The incident is being treated as a grave and criminal assault.
Anyone with information on what happened is asked to call police on 612612 or Crimestoppers anonymously on 0800 555 111.

Princess Anne opens Jersey Zoo gorilla enclosure
Tarifa Jet and Caesarea Trader return to service
Jersey Post returns to profit
Could harnessing the youth vote change the election?
Inclusive island-wide fundraising challenge asks for 'moment of understanding'
Jersey ranks second in list of short-break destinations
HMS Tyne visits for Jersey Boat Show
Penalty shootout ends Jersey Bulls' promotion hopes