Some 1,500 pupils at Wennströmska School were ordered by the principal on Friday morning to leave the school grounds.
Police would not reveal the exact nature of the threat, other than to say that it hade been made via MSN Messenger.
“We have not identified a suspect and are trying to trace the source of the threat. For the purposes of the investigation we can’t say what was relayed,” said police spokesman Ulf Palm.
Staff at the school took a decision on Friday morning to keep the school closed until Tuesday. All pupils were told to leave and the school was evacuated in an undramatic fashion.
Principal Marianne Thorén-Andersson said that the school had been made aware of the threat on Thursday.
“Quite late yesterday afternoon a pupil said that he had a contact on MSN and had read that the school would come under threat in some way.”
According to Thorén-Andersson, the threat was quite vague, with no specific details given regarding the sort of attack the school might face.
“We then contacted the police. Since then we have been in talks with the police as well as discussing the matter here. This morning all members of staff were called to a meeting and we explained that we wanted to close the school for security reasons.
“After that we told the pupils about the threat and said that we had decided for safety reasons to close the school today and Monday. The police need to be able to work on this in peace and quiet,” said Thorén-Andersson.