The Master Singers: How Highway Code single hit Top 40
The sound the pupils could hear from behind the closed doors of the music room was their teachers rehearsing the harmonies of a beautiful Anglican chant.
But upon closer inspection the lyrics to this song were different. They were taken verbatim from The Highway Code.
John Horrex's decision to marry mundane words with spine-tingling church melodies would launch one of the most peculiar musical sensations of the 1960s.
The Master Singers would work with Beatles producer George Martin, appear on records by Peter Sellers and Cliff Richard, and perform in front of royalty.
They were also full time teachers at Abingdon School, Oxfordshire. Fifty years on the school has unearthed recordings of the unlikely chart climbers.
'Smooth and cultured'
According to school archivist Sarah Wearne, to pupils, the school's Head of Physics and group founder John Horrex "epitomised the word 'smooth', being well dressed, cultured and good looking".
To celebrate the school's 400th anniversary in 1963, the counter-tenor recruited colleagues Geoff Keating, George Pratt and Barry Montague to sing his unorthodox version of The Highway Code in front of Princess Margaret.
Mr Keating, who is now 76 and lives in Galloway, Scotland, said: "He liked singing psalm chants to daft words, so in a sense the rest of us inherited his show.
"There was a great deal of laughter. The whole thing was just an enormous hoot for us."
Two years later the recording materialised on BBC radio show This Time of Day and gained popularity.
The Sunday Telegraph's radio critic said it put the programme "on the map", and Parlophone records stepped in, offering The Master Singers the chance to make a single.
George Pratt was organist at the school at the time. Now 78, he is a retired professor living near Exeter.
"We liked George Martin from the start," he said.
"He was clearly very, very high-powered. I suggested a string quartet and bassoon for a song arrangement I'd written... and was awed when he turned to his secretary and said 'Get me a string quartet and bassoon for 9.30 on Monday morning'."
The Highway Code charted at number 25 in May 1966, outselling singles by The Kinks and Bob Dylan.
Realising they were onto a winning formula, their version of The Weather Forecast followed in October.
"It was just a pastime for us," Mr Keating said. "We were serious schoolmasters getting on with our jobs. We had no ideas of breaking into the pop world but somehow we almost accidentally did."
Ms Wearne said: "James Cobban, the headmaster, definitely took it in his stride, noting publicly the group's success and their television appearance on Ken Dodd no less."
The group was also handpicked to sing with Peter Sellers.
As arranged by Geoff Keating, the actor's cover of The Beatles' Help! became a Victorian-style hymn.
Less successful was their collaboration with Cliff Richard on his 1967 Carol Singers EP.
"He's not a great singer," believes Mr Keating. "He had a very small vocal range and so it was quite difficult to write things for him.
"The songs weren't very brilliant I must say. We're not very proud of them."
Prof Pratt added: "Cliff was self-confessedly musically illiterate but had a phenomenal musical memory.
"I remember correcting one note he was singing wrong, and he simply thought about it for a couple of seconds and then locked it into his mind."
Eventually Barry Montague moved to Australia, and was replaced by Mike Warrington.
The other teachers were also moving on from Abingdon.
John Horrex became head of physics at Glasgow Academy, Prof Pratt took up a post at Keele University, and Mr Keating taught at Cheadle Hulme School in Cheshire.
However, their TV appearances increased, including a regular stint on an ITV variety show where they sang melodies to whatever the producers thought up each week.
Mr Keating remembers it as "nightmarish because by that time we'd dispersed".
"In the end we heard one of the recordings that we'd done with no rehearsal which had been put together in two minutes.
"We didn't like what we heard so we gave up doing it."
"I simply kept my head down, and few knew," Prof Pratt said of his life following the break up.
"I was a very self-consciously serious academic at that time as one is when starting a university career."
But the music had a life of its own.
Michael Bates, a fan from Oklahoma, first heard The Weather Forecast on an American radio station in the 1990s.
"The combination of a mundane text like a weather forecast with the sublime interwoven harmonies of Anglican chant were an irresistible combination to me," he said.
For Geoff Keating's golden wedding anniversary to wife Helen, The Master Singers reunited one last time in 2009.
John Horrex had retired to Canterbury, and until recently was still involved in his church choir and a barber shop quartet.
He died of cancer on 18 March. The Weather Forecast was performed at his funeral.
Ms Wearne describes him as a "huge figure" at Abingdon School.
Recently unearthed recordings there include a version of The Beatles' From Me to You, sang by the pupils in Latin, underlining the sense of fun he brought to learning.
Former pupil Alastair Tainsh said despite the novelty factor of having a teacher with chart success, he remembers John Horrex as the first to get him interested in physics, crediting him with the A he achieved at O Level.
"He was a very good teacher who could interest all pupils in his subject," he said.
So while The Master Singers may not have had the archetypical pop career or lasting success, perhaps in more important ways their legacy lives on.