She was found guilty at Winchester Crown Court today.
Earlier the court heard the teenager, who cannot be named for legal reasons, lured her friend to a quiet part of the grounds at the start of a school day, telling her she was going to give her a present.
But James Newton-Price QC, prosecuting, told the jury that the defendant asked her friend to close her eyes and hold out her hand before pulling out a kitchen knife and stabbing her in the chest.
The girl escaped with a minor injury because she opened her eyes as the defendant lunged at her, the prosecutor said.
The court has heard that the defendant was obsessed with serial killers and school shootings and had researched fatal stab wounds on the night before the attack on April 25.
She had also previously planned to kill her mother and brother and created a “kill list” of the people she would target in a school shooting, the court has heard.
The defendant admits stabbing her friend but denies that she intended to kill her.
She had chosen the girl as her target because she felt “uneasy” about her after fake social media profiles were set up in her name the previous summer.
The defendant, now 15, who had researched fatal stab wounds, denied intending to kill her victim but had admitted wanting to hurt her.
She had previously pleaded guilty to unlawful wounding and possession of a bladed article on school premises.