The three teens — two boys aged 14 and 15 and a 16-year-old girl — face charges of conspiracy to commit murder and will remain detained until a bail hearing Monday.
They have pleaded not guilty to the charges.
Police arrested the three early Thursday morning after receiving a phone call from the administration at Le Sommet, a high school in a Quebec City suburb.
Quebec City police spokeswoman Catherine Viel says the three are accused of hatching a plan on Facebook — allegedly a clear plan to cause the deaths of many people at the school.
The alleged plot was directed at school administrators, teachers and students.
After questioning the three teens, authorities decided the serious charges were warranted.
A search of the students' lockers did not turn up any weapons and police couldn't say if they had access to any.
But the use of firearms was part of the alleged plot, Viel said, adding that the accused spoke specifically about guns and explosives online.
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