Edward Woollard, an 18-year-old sixth former, pleaded guilty to violent disorder after he was caught on camera heaving the metal fire extinguisher off the roof of Millbank Tower in London, which was occupied during the protest on 10 November.
The extinguisher fell seven floors, narrowly missing a line of police officers who were attempting to keep protesters out of the building.
The student at Brockenhurst College in Hampshire, who handed himself in to the police after admitting to his mother that he was responsible for the incident, was sentenced at Southwark Crown Court today.
Judge Geoffrey Rivlin QC told Mr Woollard it was “deeply regrettable, indeed a shocking thing, for a court to have sentence a young man such as you to a substantial term of custody”.
He added: “The right of peaceful protest is a precious one. Those who abuse it and use the occasion to indulge in serious violence must expect a lengthy sentence of immediate custody.”