Mohammed Ibrahim, of Gladstone Street, was sentenced to one year and four months on Wednesday, November 13, at Luton Crown Court.
At around 3pm on 2 May 2018, Ibrahim approached a student on Cauldwell Street and demanded he hand over his bag.
When his victim tried to reason with him, Ibrahim put him in a headlock and threatened to stab him.
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Believing Ibrahim to have a weapon, the man handed over his bag and then returned to his college to report the incident and call the police. No weapon was seen during the incident.
Detective Constable Dave Brecknock, who investigated, said: “Ibrahim's threats of violence were very real, causing genuine fear to his victim.
"His behaviour also means he has forfeited his own education and will now spend time in a Youth Offenders Institute where hopefully he will consider his future conduct.
“I also hope this sentence will serve as a warning to any young people who think robbery and violence, or the threat of violence, are acceptable, and that actions have very real consequences.”
Anyone with information about crime is asked to call 101, or 999 in an emergency.