The attack took place at Desborough Leisure Centre in Ironwood Avenue some time between 5pm and 6pm on Monday, October 16.
A police spokesman said: “One attacker was a white male, aged about 16, with short brown hair and an average build.
“He had a spotty face.
“A second male was aged about 15, slim, with blond hair, one half of it shaven and the other half long.”
Anyone with information about the attack can call Northamptonshire Police on 101 or Crimestoppers on 0800 555111.