Emma Powell, a 14-year-old pupil at a fee-paying grammar school, plus friends Jade Furneaux and Gemma Sprake, also 14, carried out the attacks as they were walking along the street. Two of the incident took place within five minutes of each other.
The victims included three 17-year-old girls in Horndean, Hampshire, and three more girls in nearby Havant. They also carried out an unsuccessful robbery on two teenage boys.
Powell was from a council estate and had won a place as a pupil at the pounds 2,000-a-term Portsmouth Grammar School. She was taking 10 GCSEs and was described by her barrister as having "grade A" potential.
Judge Roger Shawcross lifted an order which protected the girls' identity and told Portsmouth Crown Court that the public had the right to know who they were because the attacks were so serious.
The trio will now spend their detention in local authority secure accommodation or in a young offenders institution.
Sentencing them Judge Shawcross said: "You three were involved in a short but extremely unpleasant campaign of robbery.
"You targeted young people walking along the streets minding their own business."
He said the public was entitled to walk the streets in safety and that he would be failing in his duty if he did not pass a custodial sentence.
Robin Leach, for the prosecution, said Sprake had threatened people with a knife and the trio had shouted out: "Yeah, we did it", after the Horndean robbery. Matthew Farmer, for Powell, said his client had fallen in with the wrong crowd and "gone off the rails." She came originally from a housing estate in a run-down area and had shown some signs of rebellion after feeling different from others at her school. He said: "Emma came from a council estate in Portsmouth. She was on an assisted place at the grammar school."
Furneaux, of Portsmouth, and Sprake, of Coventry, both now 15, admitted five charges of robbery and one of attempted robbery. Powell, also now 15, had been found guilty by a jury of four charges of robbery. Sprake was ordered to be detained for two years and nine months, Powell for two years and six months, and Furneaux for two years.
After the hearing the officer in the case, Detective Constable Gary Hill, said that when dealt with by police officers the three teenagers were courteous and polite. But he was unable to explain what had led them to carry out the terrifying robberies on their young victims. "It's unbelievable that a group of young people particularly young girls could be responsible for such offences."
He added: "It's a mystery why these girls should have got together apart from the area where they live. There's no obvious connection.
"The victims of the offences in two cases were particularly young girls.
"They were very shaken by their experience. They just want to put that behind them."
It is usually illegal to publish the names of young people involved in court cases. But Judge Shawcross took the unusual step of lifting the ban, a Section 39 order under the Children's and Young Persons Act 1933, after an application by the local newspaper.Reuse content