Christopher M. Plaskon was arrested April 25 after witnesses at the school said they saw him push 16-year-old Maren Sanchez down the stairs then stab her in the neck with a kitchen knife.
Sanchez died from the injuries and some students suspected the cause for the violence was because she turned Plaskon down for a prom date.
Plaskon allegedly told School Resource Officer James Kiely, "I did it. Just arrest me."
Plaskon appeared in court for the first time Friday, the same day Sanchez was buried. He was charged with murder as an adult, but placed in the Manson Youth Institute in Cheshire, Conn.
Doctors determined Plaskon is mentally ill, his attorney, Richard T. Meehan Jr. said.
He is “still displaying signs of active psychosis,” Meehan said. While at the youth facility, he will be placed under "one-to-one suicide watch," he added.
Plaskon is being held in lieu of $3 million bail. Should he be found guilty, he faces up to 60 years in prison.