A security guard has been posted at Lyneham High School after an outbreak of trouble there earlier in the week.
It's understood that two girls and a boy were sent home. Police were called and the ACT education directorate then decided to employ the guard to keep unwanted people off the premises.
The guard from Canberra company Leader Security was stationed outside the school on Wednesday but declined to talk to the media.
The latest incident was the second serious incident at the school.
In July, police were called there after a person with a weapon, which turned out to be a replica, was on school grounds.
"The persons involved have been spoken to appropriately by police, and have been released into the care of the parents pending further inquiries. No charges have been laid at this time," police said at the time.
The education department said on Wednesday about the latest incident that "Lyneham High School was placed into a precautionary lockdown yesterday due to an incident at the school.
"ACT Policing attended the site promptly and stayed until the incident was safely resolved. The lockdown was lifted by the school shortly after."
The education directorate said that the guard would be on duty at the school "until further notice". It was working with the school on a longer-term solution to the problem.
The guard would "help manage any unwelcome visitors that may attempt to come onto school grounds".
Earlier in the year, a report said that Australian classrooms were some of the most disruptive in the world. Students reported high levels of bullying. Some felt, according to the report, unsafe at school.
The data collected by the Program for International Student Assessment (PISA) showed Australia had one of the worst disciplinary climates compared to countries that had better or similar performance in the 2022 test.
Two-fifths of Australian students said there was noise and disorder in most classes and that students get distracted by using devices such as smartphones, websites and apps, or they were distracted by peers using these devices.
Public schools in the ACT have banned mobile phones, and some teachers reported that discipline had since improved.
