I think it has to do with a provision in the constitution that services must be provided in the "minority" language where numbers warrant.
I found it in the Charter of Rights and Freedoms. Before that, and especially because education is under provincial jurisdiction, I don't think anything was guaranteed. For example, in Manitoba, in the beginning, there was French education, and the school board had a Catholic and Protestant Branch. The repeal in 1890 of the Manitoba schools act meant that the Catholic (and usually French) schools no longer qualified for government funding. Then in 1916 French-language education was abolished in Manitoba altogether. Tiny steps toward re-instating French were made on and off until 1970 when French and English were given equal status in Manitoba schools. When we learned about this (in French immersion school) it was always cited as one of the reasons that there was French-English animosity in Manitoba.
BTW, all those dates weren't just in my head. (It's been too long.) I got them from http://www.franco-manitobain.org/sfm/livret/en003.html