The other possibility is that he went overseas prior to 1939. There are many people by the name of Robert Roberts, or Robert W Roberts, b 1892, shown on the UK outward passenger lists prior to 1939. There is one for Robert W Roberts, b 1892, who sailed from Liverpool to Halifax, Nova Scotia in 1919. A Robert W Roberts, b1892, is shown on a US List of Alien arrivals at Canadian Atlantic and Pacific seaports in 1919.
He could have moved to another area in the UK which would make it difficult to trace him as well. Had this been the case, he should be traced on the 1939 register but people do get missed off at times.
It would be a help if Jeff could contact the person/people who have him in their Family Tree on Ancestry. I am on Genes Reunited and have searched that site but no one has Robert, or Ethel, in their trees.
The dates of birth in the school registers seem to be incorrrect very often from what I have seen of them.
Helig