Yes that is the ‘mystery school’. I had come across that picture on the web as being a possible candidate. Much later I found a pre-war aerial photo which nailed it down as the school. Because the Oval is a distinctive feature to find on maps I tracked back to see how the area developed. The attached pdf shows the time line of the many changes. It is still frustrating that the period between the school being a ruin in the 1950s and then the Tesco store appearing isn’t documented anywhere that I could find. Except for a couple of sparse references to a ‘strange looking’ building that seems to have been a school equipment repository of some sort, there seem to be no official documents or photos. If anyone can still fill in the gaps then that would solve the puzzle. There should be official documents somewhere leading up to Tesco acquiring the land and maybe a local resident worked in whatever building stood there. Perhaps a display board in the entrance of Tesco showing some of the history of that piece of land may trigger some more residents from the 60s, 70s and 80s to fill in some of the gaps of our knowledge.
Mystery School-1.pdf - Google Drive