Liam Brown and Kieran Mason attacked John Howson outside the brave victim's £500,000 home in Coatbridge, North Lanarkshire last October.
The pair left the 57- year-old for dead as Mr Howson's frantic lawyer wife dialled 999.
He was rushed to hospital and was lucky to survive the vicious knifing.
Brown, 19, and Mason, 16, yesterday appeared at the High Court in Glasgow accused of attempted murder.
The duo instead pleaded guilty to a reduced charge of assaulting Mr Howson to the danger of his life.
They were remanded in custody and face a lengthy spell behind bars when they return to the dock next month.
The court heard how the pair along with another male had been prowling the streets in the early hours of October 3 last year having earlier taken alcohol or Valium pills.
The accused and their friend were then spotted by residents trying to break into cars in the Blairhill area of the North Lanarkshire town.
Mr Howson's wife Aileen alerted her husband and he got dressed to investigate what was happening.
The chemistry teacher – who worked part-time at the town's St Ambrose High – was punched to the ground in an initial attack, then beaten again.
Before he could scramble to safety, he was stabbed with a 6in blade.
A blood-soaked Mr Howson shouted for his wife as the thugs fled.
He was taken to hospital by ambulance, and was later discharged.
Brown, of Maybole Place, and Mason, of Dalveen Quadrant, both Coatbridge, were later quizzed by police, but initially denied being involved in the attack.
Judge Lord Jones remanded both in custody and deferred sentencing.