All nine of the students were charged with making illegal threats, with two additionally charged with causing actual bodily harm (vållande till kroppsskada).
The school, which is the alma matter of Prince Carl Philip, was forced to close its doors temporarily following the hazing scandal, which saw one 14-year-old boy taken to hospital as his burns were so serious.
Police had initially labelled the crime as aggravated abuse.
The Swedish Prosecution Authority (Åklagarmyndigheten) wrote in a statement on Tuesday that the charges were filed after the students harassed four younger students and threatened to burn them with the iron.
"In my opinion, the threat was such that the plaintiffs felt serious fear for their personal safety. In any case, it was with ruthless behaviour that they molested these boys, all of whom were new at the school," Vice Chief Prosecutor Niclas Wargren said in a statement.
Besides the charged boys, all of whom were born in 1995, the school's house supervisor was also charged for complicity, as he knew about the hazing plan and had allowed it.
During the incident, the victims were told to lie on the floor and were made to believe they were about to be burnt with a hot iron. Some of them were then burned on their backs after one of the attackers said "Now this is going to hurt". The students responsible claimed they didn't intend to burn their fellow students, rather that they didn't think to check if the iron was still warm after it had been unplugged.
The incident did not mark the first time Lundsberg made headlines for students behaving badly. In May last year, students at the school spoke out after being forced into oral sex and eating manure and in 2011, a student had their nipples burned with an electric fly swatter.
Principal Staffan Hörnberg was sacked in the wake of the hazing scandal. The prestigious school reopened in early September and the decision to close it in the first place was criticized by an administrative court (förvaltningsrätten).