A teenager terrified four schoolboys during a knifepoint robbery outside the Corn Exchange in Leeds city centre.
Sery Tokro was locked up for four years over the incident in which he took two mobile phones and a watch from his victims.
Tokro and a 17-year-old accomplice, who cannot be identified, targeted the group of boys around 3pm on August 20 this year.
Ian Cook, prosecuting, told Leeds Crown Court how Tokro took out a knife and pointed it at them
Two of the victims immediately fled while the others remained "frozen in fear".
Tokro told the remaining boys they were getting robbed.
The victims handed over two mobiles phones and a watch.
One of the victims had to unlock his phone using his thumbprint after Tokro threatened to stab him.
Tokro pleaded guilty to two counts of robbery and two counts of attempted robbery.
The 17-year-old pleaded guilty to two counts of robbery and a further charge of common assault in relation to an attack carried out on a taxi driver on September 22 last year.
Tokro has nine previous convictions for 19 offences, including robbery, battery and dangerous driving.
The youth has three previous convictions for five offences, including for robbery and conspiracy to supply Class A drugs.
Richard Reed, mitigating for Tokro, said his client committed the offences after being released from prison.
Mr Reed said Tokro was desperate as he had nowhere to live and no money for food.
Mark Foley, mitigating for the youth, said his client committed the offences when he "got into the wrong crowd" and started using cannabis.
Sentencing, Judge Robin Mairs said: "I have no doubt those young boys were utterly terrified.
"The lad was robbed, not just of his belongings - his phone - he was robbed of his sense of security.
"Both of you have worrying previous convictions.
"The courts have bent over backwards with warnings to avoid custody.
"A custodial sentence can be avoided no longer."
Tokro was sentenced to four years' detention in a young offenders institution.
The youth was sentenced to two years and three months in custody.