Two yobs caused £150,000 of damage to the education block at a young offenders' institute after a minor classroom incident, a court heard.

Ashley Howard and Michael Healey barricaded a classroom at Cookham Wood, in Rochester, smashed a hole in the ceiling and climbed in.

Over several hours they wreaked havoc, smashing furniture, windows and air conditioning units, before riot squad officers restored order.

Maidstone Crown Court heard that to install extra security at the block to prevent it happening again, the cost would rise to £240,000.

Howard, of Jefferson Road, Sheerness, and Healey, of no fixed address, admitted destroying property. Healey also admitted affray.

Healey, 18, was sentenced to 21 months youth custody and 17-year-old Howard to 12 months. Both had a long list of previous convictions.

Anne Phillips, prosecuting, said trouble started after Healey took a pack of playing cards into the classroom on October 13 last year and was told they would be confiscated if he did not put them away.

Howard accused the female teacher of being out of order and disrupted the class. He was told to leave the room to calm down, but he refused.

A prison officer removed him, but a short while later he was allowed to return.

He continued to be disruptive and while the teacher had her back turned a pen was thrown at her.

The inmates were told the class would be closed if the culprit did not own up. Healey picked up a table and swung it. The teacher left the room.

Healey left and then returned and barricaded the door. He made a hole in the ceiling and climbed into it with Howard.

Over the next 10 hours they trashed seven of the eight classrooms in the block. In the art room, paint was thrown at riot officers and small fires were lit.

Recorder Simon Crookenden QC watched extensive CCTV film of the incident, showing the teenagers' wrecking spree and taunts to officers.

Howard assaulted a prison officer, kneeing him in the face, the next day in the segregation unit. He was sentenced to four months custody for assault.

Recorder Crookenden said Healey was the ringleader and brandished an iron bar at officers trying to get to him. Howard was a willing assistant.

"A very substantial amount of damage was done," he said. "The classrooms have been out of action since. This was a very serious incident."

Gabrielle Moore, for Howard, said the teenager went into foster care at the age of 13 and he was "lured into the criminal justice system" through lack of support.