The 14-year-old boy, who cannot be named because of his age, admitted a charge of possessing an offensive weapon.
He appeared at Liverpool Crown Court on Friday.
The schoolgirl was fatally stabbed in the neck following a Christmas lights switch-on in Liverpool city centre on November 25.
The teenager, who cannot be named for legal reasons, appeared via videolink at Liverpool Crown Court on Friday charged with her murder.
He admitted possessing an offensive weapon, a knife, in Church Alley, Liverpool, on November 25.
About a dozen members of Ava’s family were in court for the hearing, which lasted about half an hour, and some were in tears as the teenager entered his pleas.
Judge Mrs Justice Yip told the defendant: “Your trial is going to take place in May, starting May 9.”
The case is expected to last three weeks.
Ava, who has been described as “popular” and “bright”, was assaulted in the city centre just after 8.30pm.
She was taken to Alder Hey Children’s Hospital after paramedics arrived, but died a short time later.
After her death, her father, Robert Martin, said the family were “completely devastated and heartbroken”.
Three other boys were also arrested following Ava’s death.
A Merseyside Police spokesman said a 16-year-old boy was released on bail, a 14-year-old was released under investigation and no further action was taken against a 13-year-old.