The ACT Education Directorate said a small group of students had distributed inappropriate material to thousands of their peers on Friday, as police investigate the incident.
Student email and the Google learning platform were closed on Friday afternoon and are expected to be back up this week.
ACT Education Minister Yvette Berry said a student had accidentally used a group email code and sent their work to every year 8 student in the public system.
"From there, other students used the year 8 email distribution list to share content. Some students quickly figured out how to email other year groups using the year 8 email distribution list code," she said.
"A small number of students shared inappropriate material, including pornographic images, using the email distribution lists."
Ms Berry said no external body had hacked or exported information from the system.
Education Directorate deputy director general David Matthews said police were investigating the incident to determine if the "[fewer] than 10 students" who had distributed material had committed an offence.
The eSafety Commissioner has also been notified and the Education Directorate will undertake an independent audit.
Mr Matthews said it was an "distressing and extremely regrettable" incident.
"We obviously have a complete audit log of what occurred on Friday in terms of the less than 10 students that actually uploaded the material in the first instance," he told ABC Radio Canberra.
"We also have a log of how those emails were then distributed through the system."
"The opportunistic nature of this shows a lot of the young people didn't have a lot of time to think and certainly they weren't planning.
"But they did have access to inappropriate material that they then deliberately posted through these email groups which is a serious act and we'll certainly be engaging with those young people on that basis.
"We'll be talking to each one of those students today and we'll be doing further forensic work to identify who distributed the material from there."
Parents and students are asked not to share the emails and to delete any copies they may have.
Students are expected to regain access to Google Drive and Google Classroom by Monday afternoon.
"The directorate is also working to remove all inappropriate material from email accounts and further ensure that students cannot access distribution lists," Ms Berry said.
Students are expected to have access to their email accounts by the end of this week.