Teachers Fiona Elias and Liz Hopkin and a pupil at Ysgol Dyffryn Aman in Ammanford, Carmarthenshire, were taken to hospital after being stabbed on 24 April.
The girl, who cannot be named due to her age, admitted the triple stabbing but denied attempted murder and will face a retrial next year.
The trial had been taking place at Swansea Crown Court this week, with the juror being sent out for deliberations on Tuesday (8 October).
Judge Paul Thomas KC discharged the jury and said:
“There has been a great irregularity in the jury which we all agree has irretrievably compromised our ability to consider this matter.
“With the greatest of possible reluctance, I will have to discharge this jury.
“The consequences of that are profound and unsatisfactory, leaving a 14-year-old girl to stand trial again in the new year.”
