I'm not sure where in Montreal other than Mount Royal Park you'd find a forest large enough for mechas to trample through; maybe at the western tip of Montreal island, somewhere around Senneville and western Pierrefonds up to about Cap-Saint-Jacques. The school could be somewhere north of the city proper, like in the Laurentian foothills north of Mirabel Airport (which still exists for cargo flights). I know that it's set a couple of centuries in the future (although people still drive Porsche 911's from around the year 2000) which gives nature plenty of time to reclaim some of the suburban areas.
ADDENDUM: Come to think of it, if the training ground is a few kilometres away from the school proper, Senneville would be a pretty good candidate as there is a large campus to the south, Macdonald Campus in Sainte Anne de Bellevue, which has both John Abbott College and the western campus of McGill University which is largely focused on agriculture and natural sciences. Its got acres of farmland on which they could build École du Ciel in the future. I'm still leaning more towards the school being in the Laurentian foothills due to the pine trees and hills but Senneville is at least plausible considering I don't think even the future Montreal mayor would want to close Mount Royal Park to make it a mecha training ground.