If you ever bury a time capsule, make sure you leave a map of exactly where it is.
The capsule was to mark the 150th year since the foundation of the school in 1846.
By then, the school was already housed in its second building erected in 1912.
More recently, last September, the school moved again to brand new accommodation a few hundred yards away on the other side of the A25 Maidstone Road.
As work began to demolish the old building, Mr Wishart did his best - with the full permission of the contractors Baxhall - to save as many items of historic interest as possible.
He said: "I managed to rescue the school bell, which is great because it was actually the bell from the original 1846 building. It is now on a wall of the new school, and is the new school's only real connection to its origins. It's marvellous to think of the bell still ringing out the end to playtime after 176 years."
He was also able to rescue some mosaics made by the pupils in 2012 to mark the centenary of that school building. They too have been removed and now hang near the entrance to the new school.
However, Mr Wishart failed to find the time capsule. He said: "I found a newspaper report of the era recording its burial, and when I looked up in the school log there was a mention of it, but not where it was."
The time capsule contained newspapers from the day, sweet wrappers from the tuck shop and handwritten accounts by the pupils of what their daily life was like.
Several people suggested to him it was near the lower school outside reception, but no matter how many holes he dug he couldn't find it.
He said: "I even appealed on social media for past pupils or parents who might remember. Several people said they thought it had been moved shortly after the first burial, but no-one knew quite where."
Baxall has now finished the demolition and the cleared site has been boarded off waiting for KCC to find a purchaser.
Mr Wishart said: "My search has come to an end because I no longer have access.
"I can only hope that when the developers move in they will keep an eye out for it.
And he added whimsically: "There was one large slab of concrete that had been the base for a shed that wasn't broken up.
"I'm just wondering if it is under that ... "
If anyone wants to know more about the history of the three Platt schools, Mr Wishart will be giving a talk on the subject to the Platt Society in Platt Memorial Hall on Tuesday, February 22 at 7.30pm. Admission is £2.50. All welcome.