Sussex Police investigating reports of shots being fired at a college say they are liaising with counter-terrorism officers.
Two members of staff at Crawley College have been hurt following the incident in West Sussex.
They were treated for minor injuries at a walk-in medical centre, which were not gunshot wounds.
Sussex Police said it received multiple phone calls shortly after 3.10pm and armed officers were quickly on the scene.
Pupils and staff were evacuated to a safe space while police searched the buildings.
An 18-year-old man from Crawley has been detained and remains in custody. A firearm and a knife have been seized.
There is a large police presence and people are being urged to stay away from the area.
Officers are appealing for anyone with information about the incident to come forward.
It comes as a student praised his teacher who barricaded a door and kept his class calm.
James, who did not wish to share his second name, was on the sixth floor of the building.
"I was doing my work at the time then suddenly heard some gunshots from the very bottom of the tower," the 19-year-old said.
James said police arrived just a few minutes later and although the experience was scary he "felt calm and safe" thanks to his teacher and the fast police response.
"It’s a really new experience and felt really scary due to there being a gun around where I’m doing my college work," he said
"The teacher, he barricaded the room’s door just in case and I have a lot of trust in him, since we were far up the tower I felt safer too."
Chief Superintendent Howard Hodges said: "Two staff members at the college suffered minor injuries and presented themselves to a walk-in medical centre.
"No one else has presented themselves to us with injuries but if you need medical assistance please contact the ambulance service or speak to an officer.
"We are liaising with officers at Counter Terrorism Policing South East (CTPSE). All lines of enquiry are open at this stage.
"Roads in the vicinity of the college have been closed and we continue to ask members of the public to stay away from the area at this time."
Crawley College said it was not aware of any serious injuries after the incident.
The college tweeted: "We are aware of an incident at Crawley College this afternoon. Sussex Police are present on the scene and a man has been detained.
"We are awaiting further information from the police. We are not aware of any serious injuries. Our priority remains the safety of students and staff."