It is constitutional too, not just in Newf. It's the system I grew up with in Quebec. I was in the Protestant school board in the West Island; across town (Pointe-Claire), St. Thomas High School was the Catholic high school, and it was effectively two separate schools, one French, one English. Naturally all the Jewish, Muslim and Hindu kids ended up in the Protestant schools (no religious studies, for one).
Quebec has since got smart, I think. Now the split between school boards is purely linguistic. It took a constitutional amendment to get it done, however.