Miss Lancaster was begging the gang to stop beating Robert Maltby, 21, when they turned on her in Stubbylee Park in Bacup, Lancashire, the jury was told.
The court heard Miss Lancaster's facial injuries were so severe, paramedics did not know what sex she was.
Tests indicated she had been kicked and stamped to death, with the pattern of some footwear still on her head.
As Miss Lancaster kneeled down, cradling her boyfriend's head on her lap and calling for help, the accused and the then 15-year-old youth who has already pleaded guilty to murder, turned on her.
The second boy kicked her in the head, with the accused joining in, the court heard.
Paramedics found the couple lying side by side, covered in blood and unconscious.