One of Brianna Ghey's killers, Scarlett Jenkinson, had previously drugged a younger student with a cannabis sweet before being transferred to Brianna's school.
The victim's parents did not support further police action regarding the drugging incident, leading to the matter being passed to the school for support.
Jenkinson and her accomplice, Eddie Ratcliffe, had discussed their plans to commit murder shortly after Jenkinson transferred to her new school.