The Toronto Public Library has photo of a "Weston High School" located on the south side of King Street, west of Elm Street, not far from my parents' former house on Joseph Street. I had never known of a school having been at that location and that piqued my curiosity.
Cruickshank and Nason's "History of Weston" has the school "about a quarter-mile from the Main Street" which would put it closer to Rosemount Ave. than to Elm St. To resolve the location, I looked at a 1913 map that showed structures:
The red brick building in the middle of the above is my candidate for the Grammar School. Look at all the empty lots around it. To further convince myself:
Note that there is a slight forward protrusion of the left part of the building which matches the outline in the map. Finally, a 1924 map shows that the building is no longer there. Looking at the location today, one would see this.