She says that her agency has inspected the school before, and that it is now clear that as soon as the inspections finished, the bullying began again.
The general director says they do not see the recent suspected assault as an isolated incident, but as part of a wider culture, set of values and a world view that the school has failed to reform.
Sofia Orre is a member of the school board and former leader of the parents' association. She says to news agency TT that her first reaction is that "it is a catastrophe for the pupils who are happy at the school. It is devastating for the pupils."
The head of the teachers union welcomes the decision, "the School Inspectorate has finally shown some strength," says Eva-Lis Sirén.
Former pupils of the boarding school include members of the royal family, including Prince Carl Philip.
General director Ann-Marie Begler says that the school will only be allowed to open when it can guarantee the children's safety.
She says that those who go to the school live there 24 hours a day, and so those who are the target of bullies live with abuse, without access to parents or other forms of outside support.
The general director also says that after earlier inspections the school decided to work against its culture of traditional bullying and hazing rituals. But that the recent burning shows that the school has not managed to carry out its planned measures.
The school must close from tomorrow, for a period no longer than six months. There are around 200 pupils currently at the school. The pupils' local municipality must now offer them places in local schools, in order to comply with the Swedish school law.
The Inspectorate want Lundsberg to undertake a number of reforms, in order to be allowed to open again.
The Inspectorate also has the power to take away the school's permit to operate, and Ann-Marie Begler says that this snap closure can be seen as the first step to a permanent closure.
Before this, the Schools Inspectorate has closed only three schools, all of them so called free-schools.