Dragonskolan has around 2,000 high school students and is one of Sweden's largest high schools.
During Tuesday, staff alerted the police about a suspected threat that was spread on social media.
A teenager was arrested and questioned on Tuesday.
The person was later released but is still a suspect.
The incident is classified as serious illegal threats and serious weapons offences.
Hear from the area manager of the high school and students about the incident in the video above.