Martin Backhouse, 61, discovered 913 gold coins neatly stacked in dusty hand-stitched packages and pouches beneath the instrument's keyboard base while he was in the process of tuning it.
Speaking to the Shropshire Star, Backhouse said: "I had started on the piano and was just giving it a general overhaul.
"As I did so I saw a small package. At first I thought it was moth balls.
"Then I saw the gold. I thought it was serious so I went and saw the school's business manager and the headmaster and told them I needed to show them something urgently.
"We took out the rest of the packages and then we very quickly realised that we needed to contact the coroner as I knew something about possible treasure."
Backhouse is entitled to half of the hoard's value under the Treasure Act 1996.
Ruling the coins to be treasure at an inquest on Thursday, Shropshire coroner John Ellery said: "We simply do not know how they came to be concealed".
The piano was previously owned by Graham Hemmings, 72, and wife Meg, 65 who used the instrument to teach their children music. They failed to notice the valuable haul - despite owning the piano for 33 years.
The couple, who moved to Shropshire in 2015 decided to donate the instrument to local school, Bishop's Castle Community College last summer, which then decided to have it tuned. The college is also due a share of money.
The Hemmings, who aren't entitled to any share of the treasure as they are not the legal owners, have praised church-going Backhouse's honesty in immediately reporting the find.
Mrs Hemmings said: "The sadness is, it's not a complete story, they've looked and searched for the people and they unfortunately haven't come forward.
"It's an incomplete story - but it's still an exciting story."
Experts from the British Museum found the coins ranged in date from 1847 to 1915, and consist of 633 full sovereigns and 280 half-sovereigns, with the majority dating from the reign of Queen Victoria.
Asked if it ever crossed his mind to keep the money, Backhouse said: "No - I could have quite happily swapped them a brand-new piano for that one.
"But that would not have been right or proper."
According to the Guardian, Backhouse said he will give a percentage of the money to his church and as he now has tinnitus, which makes his job difficult, he was hoping for semi-retirement.
