Maz has been wrong before. Not a good photo
here, but since the illustrated McGill crest in the montage from WWI is the same as the COTC badge, and since yours is dated 1915, looks like he's wrong again re dates.
Too bad about the res. of this picture as it would be a no-brainer to ID the badges in use at the time. Maybe McGill archives could provide a better version.
As an aside, I've seen the "11a" badge for sale described as a 'cadet' badge, although it is not shown on the
Cadet history site. Found a pdf of a McGill booklet dated 1906 which says:
"The Cadet Corps is part of the Sixth Regiment (Duke of Connaught's
Own Rifles), is officered by the College boys, and possesses all
the necessary equipment for drill and rifle-shooting . It presents a smart
appearance on parade."
Whether that 'smart appearance' includes an 11a, or any, cap badge I don't know.