The court heard the teenager had a history of speaking to men online and arranging to meet then, before confronting them.
Judge Christoper Ball QC said some of her evidence in court was "plainly untrue" and her validity as a witness in any future court case must be considered.
He said: "The prosecution must give careful consideration before relying on the evidence of this witness in any pending case if they are putting her forward as a witness of truth.
"Her account of her movements and conduct in this matter falls seriously short of the truth.
"Her denials of knowledge of the use of a knife were plainly untrue."That being the case, your honour (and this is quite an extraordinary statement, isn't it?), perhaps the general public ought to be entitled to know who she is?