Police were called to a fight involving students at Campbell High School on Monday.
An ACT Policing spokesperson said officers responded to a fight about 2:25pm between some students and other teenagers who were not students at the school.
One boy was assessed by ACT Ambulance Service officers and did not require transport to hospital, the spokesman said.
Campbell High School principal John Manders told parents and carers about an unwelcome visitor to the school via email on Monday afternoon.
"I'm writing to inform you about an incident that occurred today where a person entered school grounds without permission," Mr Manders said.
"ACT Policing attended and escorted them off school grounds. No students were injured during this incident."
The email did not mention that a fight broke out or that an ambulance was called to the school.
Mr Manders said students who required support should contact the school office.
"The safety of our students and staff is always our priority and I am pleased that students and staff responded to this situation quickly and professionally," he said.
"Your student may want to talk to you about what they saw and how they are feeling. I encourage you to speak with them and ask the school if your student needs any extra support."
The ACT Education Directorate has referred questions to ACT Policing.
We've made it a whole lot easier for you to have your say. Our new comment platform requires only one log-in to access articles and to join the discussion on The Canberra Times website. Find out how to register so you can enjoy civil, friendly and engaging discussions. See our moderation policy here.