I've had an excellent experience with the courses offered at the Centre Saint-Louis. The teachers really went above and beyond, especially considering the limited means of the school commission.
It's true that there are a lot of anglophone students, and especially among the lower level classes a lot of English is spoken during the breaks. But there is only so much the school can do about that. In my mind, it's really up to everyone to kick themselves in the behind and start using their French once it is sufficient to say more than oui and non.