A teacher was seen racing out of her classroom clutching a large kitchen knife after a 15-year-old student stabbed her in the head during a terrifying attack at a school in the United Kingdom.

Shocking surveillance footage released after the teen was sentenced shows the boy calmly walking into history teacher Vicki Williams’ classroom at Milford Haven Comprehensive School in Wales before suddenly rushing back out.

Moments later, Williams emerges behind him holding the knife and rubbing her injured head.

“I thought to myself, ‘I hope that isn’t a knife because I’m dead,’” Williams later told police, according to The Sun.

“I thought I was dying. I thought I had seen my daughter for the last time.”

Williams suffered a scalp injury, defensive wounds to her hands and a nearly 3-inch gash on her back during the attack.

Catherine Wilcox, the school’s safeguarding lead, described the gruesome scene in court.

“I could just see blood, her hair was all over the place,” Wilcox said.

“She just looked horrific. Her skin was the color of a dead body.”

The teen, whose name has not been released because he was 15 at the time of the stabbing, was arrested later that same day.

Authorities said he told his grandmother that “something went in my head.”

The boy was initially charged with attempted murder but was ultimately found not guilty of that offense.

He was instead convicted of wounding with intent and sentenced Thursday to 4 1/2 years in a youth detention facility.

Judge Paul Thomas delivered a stern warning while handing down the sentence.

“You could have killed her,” Thomas told the teen. “It was pure luck that she did not have far worse injuries, maybe even fatal injuries.”

The exact reason for the attack has never been made clear.

However, the judge said there was no doubt in his mind that the teen brought the knife to school intending to seriously hurt Williams.

“What I do know for certain is that you took a knife that day to school because you wanted to really hurt her with it,” Thomas said.

Williams said the attack has changed her life and left her nervous around young men.

She also fears she may never be able to return to the classroom.

“I don’t think I could be the same teacher I was before,” she said in a victim impact statement.

“School starts in early September and at the moment I don’t think I’ll be returning.”