The jury were shown a photograph of Brianna s body, taken from the body-worn footage of one of the first police officers to arrive at the scene in Culceth Linear Park, Warrington, Cheshire in February.
The 16-year-old, who can only be referred to as Boy Y, agreed that the joke had gone too far when Brianna was stabbed 28 times and left dying face-down in the mud, jurors were told.
The court has been told that Brianna Ghey died after being stabbed 28 times in Culcheth Linear Park in Cheshire after being lured there by Boy Y and his co-accused, referred to as Girl X.
The teenager, who was 15 at the time and is identified only as girl X, was held the day after transgender Brianna, 16, was found stabbed 28 times in Culcheth Linear Park, near Warrington.