...overall, over 90% of students attend public schools of various types, and while different provinces have different approaches to running school boards (English/French, Public/Religious, etc.) the end result is a system that has a very high success rate (according to that, and many other, articles on the subject).
That's basically why private schooling has had such a slow growth rate here - there isn't a glaring need, and other than class-based isolationism, there isn't much rationale for spending that much money for a similar service you can get for free.
And yes, all the public teachers in Canada are unionized, and hold tenured positions, so I don't see how those can be the reasons for the public school system in the US failing so badly. My only 'educated' guess is that the private system in the US has drawn the best and brightest to higher paid positions, leaving the public schools with (for the most part) 2nd rate teachers.