Investigators say the girls, who were 12 at the time of the attack in 2014, plotted for months before luring their classmate into some woods after a birthday sleepover and repeatedly stabbing her.
The victim, who was also 12, was found along a road, bleeding from 19 stab wounds that nearly killed her.
The girls have been charged with attempted first-degree intentional homicide and if convicted could go to prison for up to 65 years.
Anyone 10 or older charged with first-degree attempted homicide is automatically considered an adult under Wisconsin law. But defense attorneys have argued that the case belongs in juvenile court, saying the adolescents suffer from mental illness and won't get the treatment they need in the adult prison system.
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Experts testified that one of the girls has schizophrenia and an oppositional defiant disorder that requires long-term mental health treatment.
The other girl has been diagnosed with a delusional disorder and a condition known as schizotypy, which a psychologist testified made her vulnerable to believing in Slender Man.
Citing the ruling last year, the appeals court said if the girls were found guilty in the juvenile system they would be released at age 18 with no supervision or mental health treatment.
It also noted that the evidence showed the crime was not accidental or impulsive, but planned out and violent. Given the serious nature of the offence, it would not be appropriate for the trial to take place in juvenile court, the appeals court ruled.
Kevin Osborne, the assistant Waukesha County District Attorney, said he was pleased with the decisions, but declined further comment because he has not yet read them.