BRISBANE, Sept 13 - The guard, aged in his 50s, was attacked after disturbing the intruder at Somerville House in South Brisbane about 5.30am on Wednesday.
He has knife wounds to his torso and arms and while his injuries are significant, they are not life threatening.
Police have warned members of the public to be vigilant, saying the offender may still have the knife used in the attack.
The guard was nearing the end of his shift when he noticed someone trying to break into the school's performing arts complex. When he challenged the intruder, the offender rushed at him and began stabbing.
"Upon approaching this male person, the security guard received multiple stab wounds to his arms and upper torso," Detective Inspector Tony Geary told reporters at the scene.
"It's certainly likely that the offender has blood on them, and it's quite possible that the offender is still armed."
The attacker fled on foot but it's believed he may have boarded a commuter train during his escape.
The school has a boarding house, meaning students were on the grounds when the attack occurred.
Classes will run as usual on Wednesday, with the school saying students are not in danger. But some parents say they'll stay with their children until police catch the offender.
"I won't be dropping her off until I have a little bit more information. I'd just like to know if they've found the person who attacked the security guard," the mother of one pupil told the Nine Network.
The guard has been taken to Brisbane's Princess Alexandra hospital in a stable condition and will undergo surgery.
Somerville House spokeswoman Sarah Dreaver says the school will be open and counselling will be offered to any students who need it.
"The students are not in any danger," she told ABC radio.
"Our thoughts and prayers are with the security guard and his family at this time."
Somerville House was established in 1899 and is a day and boarding school for girls from Prep to Year 12. It's also a co-educational school for pre-prep girls and boys.