The teenager had been given a sentence of three years and two months following the killing of Olly Stephens in Reading in January 2021.
Three appeal judges concluded that the sentence was unduly lenient.
They ruled the custodial term should be increased to five years.
The teenager who was 13 at the time, admitted manslaughter.
Two males aged 14, were found guilty of murder following a trial at Reading Crown Court.
One was sentenced to 13 years and the other to 12 years.
All three were sentenced in September to custody in young offender institutions.
Lady Justice Macur, Mr Justice Picken and Judge Mark Lucraft oversaw a Court of Appeal hearing in London.
It was told Olly had been lured to a field near his home, after a dispute on social media, and “ambushed”.
Barrister Tom Little QC, who represented the solicitor general Alex Chalk, argued that the girl’s original sentence of three years and two months was unduly lenient.
The appeal judges decided that Judge Heather Norton had made an error when deciding on the teens sentence.
None of those convicted can be identified for legal reasons.