The girls are charged as adults with attempted first-degree intentional homicide in connection with the May 2014 attack, which prosecutors say they plotted for months to please "Slender Man," a fictional character they read about online.
The girls, who were 12 years old at the time, are accused of luring their classmate into the woods in Waukesha, Wisconsin, stabbing her, and then leaving her there. She survived the attack and returned to school last fall, Reuters notes.
The girls' attorneys had requested their cases be moved to juvenile court, arguing they would get better mental health treatment in the juvenile system.
But after four days of hearings in May and June, Waukesha County Circuit Judge Michael Bohren ruled Monday the girls should remain in adult court, noting sending the case to juvenile court would "unduly depreciate" the offense, the Journal Sentinel reports.
The girls are expected to be return to court Aug. 21 to be arraigned, according to online court records.
BringMeTheNews is not naming the girls involved because an appeals court could still move the case to juvenile court, which is closed to the public.
If the girls are convicted in adult court, they could face up to 45 years in prison, reports note. However, if they are convicted as juveniles they could only be held until they turn 18, WBAY said.