Kim Ramchen was on Monday sentenced to 15 months' imprisonment after admitting intentionally causing injury, assault with a weapon and unlawful assault over the December 2 stabbing.
He will be eligible for parole after eight months and 14 days, but having already spent 160 days in pre-sentence detention, could be released in August.
The 37-year-old is the son of former Price Is Right host Jacqui Ramchen, who disappeared in 1992. His late father was charged over her killing until a magistrate found there was not enough evidence to commit him to trial.
Ramchen appeared in court on Monday via video link in a green prison tracksuit and stared straight ahead throughout the sentencing.
The Keysborough College teacher had armed himself with a 10cm knife from the Melbourne school's kitchen after marking his class roll just after 3pm before barging into principal Aaron Sykes' office.
Ramchen set upon Mr Sykes, aggressively aiming the knife at his neck after learning his teaching contract would not be renewed.
As the pair wrestled to the ground, Mr Sykes yelled out for help.
Assistant principal Matthew Sloan, who came from down the hall after hearing shouting from the principal's office, was able to pull Ramchen out of the office.
The IT teacher walked off before the school was placed into lockdown.
But he returned a second time, armed with a larger chef's knife, punching Mr Sykes and holding the blade to the principal's face and neck.
Mr Sloan, who had just seen Ramchen rifling through the staff kitchen draw for the bigger knife, rushed back into the office to find the principal tangled in an office chair on the ground with the teacher on top of him.
Fearing for his life, Mr Sykes experienced a rush of adrenaline and pushed Ramchen off before his deputy pulled the teacher away and tackled him to the ground.
It took several staff to subdue him until emergency crews arrived.
Mr Sykes escaped the office and was rushed to hospital covered in blood with cuts to his jaw, face, lip and forearm.
Mr Sloan suffered cuts to his ear, cheek, jaw and ribs.
In a police interview, the former IT teacher said he had "mentally snapped" and intended to just incapacitate the principal.
Ramchen's offending was serious given it was in a school environment where students were on campus, magistrate Timothy Bourke said on Monday.
"You have attacked the school principal twice and his colleague who bravely came to his aid," he said.
"This was an unwarranted attack in a school workplace during school hours ... If not for Mr Sloan's bravery, there could've been a worse outcome for Mr Sykes."
In trying to explain his behaviour, defence barrister Amelia Beech said Ramchen had been in the midst of a "catastrophic mental health crisis" culminating from a dysfunctional childhood, challenging adulthood, stressful parenthood and undiagnosed mental ill health.
Ramchen has been diagnosed with autism, depressive disorders and anxious distress, which a forensic psychiatrist has found was "causally connected" to the offending, Ms Beech said.
At the time of the attack, his marriage was falling apart and he was unable to understand why his contract was not renewed, the defence added.
Ramchen was raised by his siblings after his mother disappeared and his father died from cancer when he was 14.
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