The 16-year-old, who can't be named due to his age, also pleaded guilty in Red Deer youth court to two breaches of probation.
A pre-sentence report has been ordered.
The teen is to remain under house arrest and in the care of his parents until he is sentenced on Oct. 3.
He is scheduled to be tried on several unrelated charges on that day.
Red Deer city RCMP were alerted on April 4 to a Facebook post threatening students and faculty at Lindsay Thurber High School.