Fucci was just 14 years old when police say he stabbed the 13-year-old cheerleader 114 times on Mothers’ Day in 2021 in St. John’s County, Florida.
As jury selection was about to begin on Monday in his first-degree murder trial, Fucci asked to address the court and said he was changing his plea to guilty, according to WJXT-TV.
“I just want to apologise to the Bailey family,” Fucci, now 16, said.
Fucci was arrested shortly after the discovery of Bailey’s body in May 2021.
He allegedly posted a selfie to Snapchat from the back of a police car after his arrest, with the caption: “Hey guys has anybody seen Tristyn lately”.
The teenagers were classmates at Patriot Oaks Academy in St. Johns.
St. Johns County Circuit Judge R. Lee Smith asked Fucci whether he needed more time to make the decision, but he declined, according to WJXT-TV.
Fucci indicated he understood the change of plea and had enough time to speak about it with his attorneys and parents.
The guilty plea had not been negotiated with prosecutors, Judge Smith said.
Prosecutors had already rejected several plea deals made by the defence, one of the teenager’s lawyers told the court.
Some of Bailey’s family members walked out of the courtroom in tears after the last-minute change of plea, according to a WJXT-TV reporter.
Prosecutors opted to try Fucci as an adult.
A report released by the St Johns County Sheriff’s Office in July 2021 revealed that Fucci had told friends that he fantasised about killing and mutilation.
After Fucci’s arrest, police found a sheath of a Buck knife in his bedroom, along with a pair of shoes and a shirt with blood.
A Buck knife retrieved from a pond near where Bailey’s body was found was consistent with her stab wounds.
According to the autopsy report, Ms Bailey had 49 defensive stab wounds on her arms and right hand.