Prosecutor Alex Chalk KC said the defendant, now 18, did not deny wielding the knife and causing Elianne’s death but had pleaded not guilty to murder on the basis that his responsibility was diminished due to his autism.
Mr Sentamu also denies illegal possession of a blade, claiming he had a “lawful reason” for carrying it.
Opening the trial, the prosecution told the court Elianne had gone along with her friend, and a group of girls, to meet the defendant in a car park on the morning of 27 September, before school.
Her friend and Mr Sentamu, who had split up 10 days earlier, had planned to exchange their belongings.
Mr Chalk told the jury that Mr Sentamu had turned up empty handed, and that Elianne was “aggrieved on her friend’s behalf”.