How interesting! I was intrigued by the envelope and after a bit of trial and error, came across the Royal Masonic School in Rickmansworth, the Junior branch of which moved to Weybridge in 1918. Could this be it? The address definitely starts with Junior and the next word could, at a stretch, be Masonic. Here's the wiki entry - the Junior school move is mentioned at 2.2 Clapham J in the contents listing http://en.wikipedia.org/wiki/Royal_Masonic_School