Morgan Geyser and Anissa Weier are charged with attempted first-degree intentional murder over the attack.
The ruling means they may face decades in jail. Their lawyers failed to get the case heard by a juvenile court.
If tried as juveniles, the pair only face a five-year term.
The victim, 12 at the time, was stabbed 19 times but survived.
She was found by a cyclist crawling from the woods where she was attacked with stab wounds to her arms, legs and torso, after the attack in May 2014.
Wisconsin Judge Michael Bohren decided the pair, now both 13, should remain in adult court after four days of hearings in May and June.
They told investigators they had stabbed their victim in "dedication" to Slender Man, a fictional horror website character.
They spoke of their desire to become the paranormal figure's "proxies" by killing to demonstrate their loyalty, police said.
Following the attack, both suspects were found walking near a local highway, and a knife was found in one of their backpacks, police said.
They said the girls planned to stab the classmate during a sleepover but instead decided to commit the crime the next morning in a nearby park.
After the murder, they reportedly intended to walk to Slender Man's mansion, which they believed was situated in Wisconsin's Nicolet National Forest.
The victim has recovered and since returned to school.

Bakewell man Stephen Downing was convicted in 1974 for killing Wendy Sewell but that was overturned in 2002.
Chris Clark said he has uncovered a crucial pathology report showing she was strangled, which was never told to the jury.
The Home Office said it would send any new criminal evidence to the police.
Evidence of police misconduct would be referred to the Independent Police Complaints Commission, the Home Office added.
Derbyshire police said the case was closed after a reinvestigation in 2002.
Mr Clark, a retired Norfolk police officer, said he has sent his findings to the Home Office after obtaining the original pathology report last month.
He said he believed the report showed Mrs Sewell had clear signs of being strangled.
She was found battered in a Bakewell cemetery in 1973 and died in hospital two days later.
Mr Downing admitted beating her with a pickaxe handle but later retracted that statement, only to be found guilty by a jury.
"I'm reporting the facts as I see them. The pathologist had evidence in his report that could have exonerated Stephen Downing," Mr Clark said.
He said the pathologist's report showed bruising on her neck consistent with a "knotted ligature" used to garrotte her and a rash in her lungs and airways, possibly caused by strangulation.
Mr Clark said none of this was used in Mr Downing's trial. He added the evidence may mean her death was linked to the Yorkshire Ripper, Peter Sutcliffe.
Mr Downing was 17 at the time of Mrs Sewell's murder but was assessed as having a mental age of 11.
His father Ray Downing and Matlock Mercury editor Don Hale campaigned for his release and in 2002 the Court of Appeal found his conviction unsafe because of "procedural reasons".
Mr Hale said the pathology report directly conflicted with the evidence the police presented at the time and called for an independent investigation into the murder.
"This information was available within two or three days of Mr Downing being arrested and it completely contradicts this so-called confession," he said.
But Derbyshire police said the murder was reinvestigated following the quashing of the conviction and it considered the matter closed.
"All possible lines of inquiry were exhausted during the re-investigation.  Twenty two people were ruled out of the enquiry but officers were unable to eliminate Stephen Downing as a suspect," a spokeswoman said.
She added there was no evidence to suggest Peter Sutcliffe - the Yorkshire Ripper - was involved.
Two 13-year-old girls accused of stabbing a classmate to please the online horror character Slender Man will be tried in an adult court, a US judge has ruled.
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A retired police officer has reported Derbyshire police to the Home Office over claims they withheld evidence in a 1973 murder case.