Cole Harbour High School was locked down for about 45 minutes on Thursday morning with RCMP on scene.
RCMP Cpl. Jennifer Clarke said police got a report of a threat by one student against another student just before 9 a.m.
A male suspect is in custody, she said. No injuries have been reported. The lockdown was lifted shortly after 9:35 a.m.
Students were kept in their classrooms and normal classes have resumed, said Doug Hadley of the Halifax Regional School Board.
"Shortly before 9 a.m. this morning, police contacted the administration of Cole Harbour High School and indicated they had information involving a threat made by one student toward another," Hadley said.
"They were in the process of trying to locate the person who'd made the threat. Neither of the individuals were in the school at the time and they advised the school to go into a lockdown until they could locate the individual because they were taking the threat very seriously."
He said the threat involved a weapon but there was no indication of why the threat was made.
There was heavy police presence at the school this morning, Grade 11 student Lauren Watton said.
"Honestly, I counted 10 to 15 cop cars here. I've seen at least one cop with quite a large gun," she said.
Hadley said the school will be contacting parents with information about what occurred this morning.