Windsor police have a male suspected of making threats against Walkerville Collegiate in custody.
The school was locked down shortly before 11 a.m. Thursday.
Windsor police allege the suspect, a 16-year-old, made a post on Twitter threatening to shoot up the school.
Glen Gervais of the Windsor Police Service said there were "very serious" threats made.
"It was a threat to cause harm to people in the school,” he said.
The lockdown lasted about an hour as police searched the high school.
"The lockdown procedure went nearly perfectly," Greater Essex District School Board spokesperson Scott Scantlebury said. "We drill on regular occasions for occasions such as this. We obviously dislike when they are real. But the drills proved their worth."
Student Joshua Lowe called the lockdown "stressful."
He said he heard rumours of a bomb or gun in the school. He said he then heard “screams from the second floor.”
“I was starting to get a little more worried,” he said. “There was a point where it sunk in that if there was a bomb or gun in the school people could die.”
Gervais said police "always have to treat it as a worst-case scenario."
"I don’t know whether any threat is ever a hoax," he said. "I consider it a credible threat when you have to lock down 900 students in a school. You inconvenienced and scared about 1,000 people. You could see the fear in the students' faces."
The suspect was taken into custody in the Bruce Avenue and Hanna Street area around noon.
Windsor Police Superintendent Mike Langlois said the incident serves to remind the public police are monitoring social media for threats and criminal activity.
Langlois claimed the tweets in question "crossed the threshold into the criminal element.”
"It’s not as simple as just ‘well, I was just joking around,’" he said. "When you’re on social media, you’re speaking to thousands, if not millions [of people].”
In this case, Langlois said it was a member of the school's staff that notified police of the tweets.