A headteacher was fined £20,000 and ordered to pay £22,000 in costs today for failing to take reasonable care for the safety of his students after a pupil fell through a skylight.
John Summerfield took the group of "slightly inebriated" sixth-formers on to the roof of Sacred Heart Catholic College in Crosby, Merseyside, when Joel Murray, then 18, tumbled 8ft (2.4m) into a corridor and fractured his skull.
The teenager also broke his ribs, perforated an eardrum and suffered permanent damage to his eye, Liverpool Crown Court was told.
Summerfield was convicted last month of breaching the Health and Safety at Work Act.
Sentencing today, Judge Nigel Gilmour QC, said it was "an isolated incident in an otherwise distinguished career".
The accident, in August 2008, happened during an evening party the school had thrown to celebrate the students' A-Level results which had been released that day.
Summerfield, 65, took about 10 or 12 students on to the roof, which was normally out of bounds, as he showed them the results of renovations to the building.
While on the roof, the trial jury were told, the principal warned the teenagers to stay away from the fragile skylight but Mr Murray placed his foot on it as he walked past and tumbled through.
Prosecutor Kevin Donnelly said: "Nobody was drunk but it is possible they were slightly affected by drink, possibly slightly inebriated.
"Mr Summerfield took a group into an area which was normally out of bounds.
"The decision was his and it was his alone.
"The very act of taking the students to that area was a breach of his duty of care.
"He failed to give appropriate consideration to the risks involved."
Mr Murray did not attend today's hearing. Judge Gilmour was told the lasting eye damage he suffered was likely to limit his employment options for the rest of his life.
Patrick Cassidy, in mitigation for Summerfield, said the teacher had enjoyed an "impeccable" 42-year career until his retirement following the accident.
Mr Cassidy said the case was an "isolated incident" which had resulted in the loss if his good character within in the education sector.
In his sentencing remarks, Judge Gilmour told the defendant: "I hope you are able to forget this isolated incident.
"This act of isolated carelessness is not a true reflection of your distinguished career.
"It was an act of folly. You didn't know the precise risk of the rooflight but you did know it was dangerous or at least potentially dangerous.
"This was a grave and uncharacteristic judgment which led to a pupil suffering serious injury.
"(For Joel Murray) the consequences have been quite serious."
Summerfield was given six months to pay the £20,000 fine, £22,708 in prosecution costs and a £15 victim surcharge.
If he does not pay he will face up to 12 months in prison.
Summerfield, of Moorland Avenue, Crosby, Merseyside, nodded to the judge as the sentence was handed down.
Mike Sebastian, the Health and Safety Executive's principal inspector in Merseyside, said: "The roof was kept out of bounds for a reason.
"As the headteacher, Mr Summerfield should have thought about the possible consequences before deciding to take them through two locked doors on to the roof.
"Students should expect to be in a safe environment when they're at school and look to their teachers for guidance on what is and what isn't safe.
"Sadly, a pupil suffered serious injuries because of the poor judgment of his headteacher, and is unlikely to ever fully recover."