The girl died after stepping in front of a passing train last week near Kumla, 15 kilometres south of Örebro.
Police began to suspect the teen’s death was a suicide after learning she had been subjected to cyberbullying.
“Everyone knew about it but nobody did anything. The teachers turned a blind eye. They should have supported her more,” a friend of the deceased girl told the Nerikes Allehande newspaper (NA).
Bullying at the girl’s school, the Vialundskolan, is a common practice according to the father of another girl who left the school after she too became a victim of bullying.
The father has now joined forces with other parents to report the school to Sweden’s Schools Inspectorate (Skolinspektionen).
“I already knew she had problems,” the man told the Svenska Dagbladet newspaper, referring to his own daughter.
He added that many other students are bullied at the school, yet no staff members seem to react.
“It can be anything from ‘You’ve got an ugly haircut’ to much worse things.”
The school’s headmaster refused to acknowledge that staff members were aware of any bullying towards the deceased 13-year-old.
Police officers have launched an investigation into the girl’s death. They initially suspected a 15-year-old boy of tormenting the victim online, but have since adjusted the focus of their investigation.
“The case of the 15-year-old boy has gone cold. We have got new indications and other suspects,” investigator Mikael Nyqvist told NA.
Sexual harassment, making illegal threats, and illegal coercion are among the possible crimes with which any guilty party may be charged.
The prosecutor, meanwhile, has promised swift action if details of the harassers emerge.
“If we get a suspect then we’ll publish their nickname immediately, but not at this stage”, prosecutor Pia Åsberg told the paper.
Sweden’s Schools Inspectorate held a crisis meeting on Wednesday morning and will launch an investigation into the school and its procedures for dealing with bullying.
A spokesperson from the inspectorate said that there has been a “lack of leadership” at the school for years.