According to documents released under freedom of information laws, police were involved in each of the incidents, two of which resulted in injuries. However, details were scant and the documents were heavily censored to protect the privacy of students involved.

The ACT branch of the union said the information did not include any critical incidents that may have happened in private schools, as they were not obliged to provide similar data.

At Canberra High School, an incident categorised as “violent/serious physical assault” involved a number students being physically and verbally assaulted. Staff were also threatened by the perpetrator.

An incident at Telopea Park School also resulted in injuries, although no details were provided.

The two incidents involving “weapons/blades and other sharp instruments” occurred at Calwell High School and Harrison School, although neither resulted in injury.

At Calwell High, other students were reported to have broken up an altercation. A teacher was reported to have “diffused the situation” at Harrison.

A spokeswoman for the education directorate said about 41,000 students attended Canberra's 86 public schools.

"The number of critical and serious incidents is small, relative to the total school population,” she said.

“All staff in public schools are trained and supported to respond to critical incidents and … school communities are informed of incidents when they occur.”

Union ACT branch secretary Glenn Fowler said parents were at risk of getting "a skewed picture of what schools are like" and there was the potential for prejudices to be confirmed.

“Any reporting of critical incidents in government schools should come with an important disclaimer: that similar information is not provided by private schools,” Mr Fowler said.

"Parents should be concerned that they are hearing only half the story.”

The Association of Independent Schools of the ACT said it did not collect data on critical incidents.

“The schools are independent by name and by nature, we don’t collect that data. We have no requirement that they provide that data. That is information for a school,” executive director Andrew Wrigley said.

He said prospective parents doing due diligence on prospective independent schools needed to weigh up the various elements of a school’s culture, including educational philosophies, pastoral care and extra-curricular activities.

“If the number of categorised critical incidents are part of that mosaic that’s important to a parent, then they should ask that question when they go and speak to the school.”

Mr Fowler suggested parents not judge any school too harshly on critical incidents.

“Schools are part of the community, and are not entirely insulated from unsavoury or, in rare occasions, dangerous activity. No school is immune, whether it is public or private,” he said.

“Statistically, the safest place for a child to be is at their school.”