Louise Allen died from a single kick to the head as she intervened in a brawl to help a friend. The two girls, then aged 12 and 13, each kicked her once to the head, Nottingham Crown Court heard.
At an earlier hearing the pair, who cannot be named for legal reasons, admitted manslaughter.
They stood impassively in the dock yesterday as Mr Justice Hidden told them: "It was wicked violence and it resulted in the totally unnecessary death of that young girl.
"I will try to make it the least possible sentence with my duty to the public. But the message must be clear to all those just as young as you two to put the boot in to someone on the ground is to commit an act which the court cannot treat lightly."
The sentence was greeted with anger by Louise's family and "extreme disappointment" by the police.
A detective involved in the inquiry said the Crown Prosecution Service would now seek legal advice from prosecuting barristers on whether to take further action.
The attack happened in last April in Corby, Northamptonshire, as group of teenagers were walking home from a fairground.Reuse content