Colleagues of a teacher who "snapped" and stabbed a school principal say they are still reeling from the attack, as details of his "catastrophic" mental health spiral emerged.

Kim Ramchen, 37, on Wednesday pleaded guilty to three charges of intentionally causing injury, assault with a weapon and unlawful assault, over the stabbing in December.

The Keysborough College teacher armed himself with a 10-centimetre knife from the Melbourne school's kitchen and set upon principal Aaron Sykes after being told his contract would not be renewed.

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As prosecutor JJ Jassar read the summary to Dandenong Magistrates Court, Ramchen sobbed into his hand over the video link.

Jassar said Ramchen entered the principal's office aggressively and aimed for Sykes' neck, with the pair struggling to the ground.

Teacher Emma Pascoe, who was in a room next door, ran over after hearing screaming and saw Ramchen standing over Sykes, swinging the knife.

She said she still experiences flashbacks to the incident and turned to alcohol to drown them out.

"The sound of students messing around and screaming 'help, help' still sets me off into panic mode," she said in a statement read to the court.

Ramchen walked off after being pulled away by assistant principal Matthew Sloan and the school was placed into lockdown.

But he returned, armed with a larger chef's knife, holding the blade to the principal's face.

Ramchen was tackled to the ground and subdued by several staff before emergency crews arrived.

School nurse Stacy Bender, who treated Sykes, recalled the look of fear in his face.

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"This has changed how I see a school environment, a place that should be safe now feels more vulnerable," Bender said in a statement to the court.

Ramchen said during a police interview he "mentally snapped" and intended to just incapacitate the principal.

"My blood just went to my head and I became incredibly angry and emotional," he said.

Defence barrister Amelia Beech said Ramchen's conduct was the culmination of substantial difficulties that had plagued his life.

He is the son of former Price Is Right host Jacqui Ramchen who disappeared in 1992, with his late father charged over her killing until a magistrate found there was not enough evidence to commit him to trial.

Ramchen had taken on a parental role within the family after his mother disappeared and his father died from cancer when he was 14.

"The lasting effects of his tragic and dysfunctional childhood, the enormous challenges of adulthood, stressful circumstances of parenthood and the debilitating effects of undiagnosed mental ill health all collided in a spectacular fashion," Beech said.

Ramchen has been diagnosed with autism, depressive disorders and anxious distress, which forensic psychiatrist Danny Sullivan found was "causally connected" to the offending, she said.

At the time, the teacher's marriage was falling apart and he was unable to understand why his contract was not renewed, the defence added.

"The offence has been committed in the context of a … catastrophic mental health crisis," she said.

Ramchen will be sentenced on Monday.

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