McKinney, 16, and Devon Erickson, 18, a male, allegedly carried out a shooting at STEM Highlands Ranch on May 7, killing one and wounding eight others.
Authorities said they initially believed McKinney was a male but later discovered she was a young female.
McKinney was being held without bond and waived the reading of formal charges.
District Attorney George Brauchler said that he will announce formal charges against McKinney and Erickson on Friday and will also announce whether he will charge McKinney as an adult. McKinney is currently being held at the Marvin W. Foote Youth Services Center in Englewood but would be transferred to Douglas County Jail if charged as an adult.
District Court Judge Theresa Slade declined a request during the hearing that McKinney's handcuffs be removed, citing what she'd read in court filings, and said she was withholding bond because "McKinney is a danger to others."
The Twitter page appeared to be taken down as of May 9. Erickson's Facebook page was removed on May 8.
A friend of both wrote on Instagram that the pair allegedly carried out the shooting because they were struggling with mental health issues.
"They’re both struggling with mental health issues and this is a time for awareness," the person claimed. McKinney "did not do this because he’s trans[gender], but had people supported him in the way he needed and deserved, he would not have struggled so much that he got pushed over the edge.”
The World Health Organization classified a person claiming to be a gender different from the one they were born as a mental illness until June 2018.
Classmates took to the page before it was taken down to denigrate the alleged shooter, with one writing: “If there is a death penalty in Colorado, I hope you get it.”