The 16-year-old was stabbed outside Player's Lounge in Yew Lane, Ecclesfield, just after 12am on Sunday, June 17.
There were a number of people in and around the bar that evening who officers are urging to come forward.
“At this time, the motive around the incident remains unclear but we do believe that the people involved were known to each other.
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Please call 101 or Crimestoppers anonymously on 0800 555111 quoting incident number 1 of 17 June 2018, with any information.