The teenager, who turned 19 this year and cannot be named as he was 16 at the time of the killing, had asked for eight to 10 years’ jail instead.
In dismissing the appeal, the Court of Appeal noted that his thought process was logical, and that he knew the nature and wrongfulness of his acts.
The three-judge court also noted that the depressed teenager had refused to seek help, and that there was no reason why the sentencing principle of retribution should not prevail in this case.
His defence lawyer said he would not be pursuing other avenues such as presidential clemency.
The teenager had attacked 13-year-old Ethan Hun Zhe Kai in a school toilet on July 19, 2021, in an unprecedented crime that shocked the nation.