Chilling CCTV footage shows the moment a teenager stalked through school corridors with a knife before stabbing his teacher.
The attack took place at Milford Haven Comprehensive School in Pembrokeshire, where the 15-year-old targeted history teacher Vicki Williams on February 5.
Footage released by the Crown Prosecution Service shows the boy walking through the corridors with the knife in his hand before the attack.
He had asked Mrs Williams to check his homework before stabbing her in a classroom at the end of the school day.
The student was found guilty of wounding with intent but cleared of attempted murder.
He admitted possession of a bladed article and was sentenced to four and a half years in youth detention on Thursday.
Christopher Rees KC, prosecuting, told Swansea Crown Court it was a "carefully planned attack."
Mr Rees said: "The defendant stabbed the victim to the back of the head. She started screaming and a struggle ensued."
Judge Paul Thomas KC said the boy had shown "considerable ill will" towards Mrs Williams and had planned the attack in advance.
Judge Thomas said: "What I do know for certain is that you took that knife to really hurt her with it that day."
He added that it was "pure luck" Mrs Williams had not been more seriously or fatally injured.
In a victim impact statement, Mrs Williams said she now suffers from post-traumatic stress disorder and has not returned to teaching.
She said: "I don’t feel I could be the teacher I was."
Mrs Williams also said she had installed cameras at her home because she no longer felt safe.
Following the sentencing, Jonathan Pritchard, senior crown prosecutor with the CPS, described the attack as "a shocking display of violence on a teacher doing their job."
He praised Mrs Williams’ bravery during the ordeal.
Mr Pritchard said: "The teacher showed great courage in what must have been a terrifying incident."
The defendant cannot be named due to his age.
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