Salut!
That's a pretty big coincidence - I just made this account to reply to another post, and your question caught my eye.
I actually attended an "all French" high school (plus junior high).
To be honest, I can't really offer you much help on the specific schools in Mississauga (i.e., I've heard neither good nor bad things about the French programs at any of the schools there).
But, here are some general guidelines.
This is what the Peel District School Board's website has to say about the public "French Immersion":
"The program is open to all students and is designed for students for whom French is a second language."
(
http://www.peelschools.org/facts/facts/french.htm)
The key phrase here being "second language." What this (likely) means is that a lot of the kids will be reluctant to fully immerse in the French atmosphere. The amount of French used will likely be minimal. And, every occasion when English can be employed (i.e., right after class is over) will be used.
So, these students will be English speakers first and foremost. French (for some) will just be an extra "burden" forced on them by their parents. The overall quality of the French education (despite what the teachers and principals will have you believe) will not be very comparable to someone growing up in France.
But, there are some
real French schools for students with French as a first language. Again, I do not know about Mississauga (and it is normally less cosmopolitan than Toronto). But, Etienne Brule (
http://etiennebrule.csdcso.on.ca/) is a "hardcore" French school. I see these kids on the bus sometimes (I live right next to the school), and even though they will talk in English, they will effortlessly switch to French at times. Their pronounciation is perfect. I.e. : they are not losing the language.
Another two "real" French schools are College Francais and Lycee Francais, both in Toronto. Lycee Francais (I think) has all the grades from kindergarten to baccalaureate. People who go to these schools have good French (due to both the schools and their French speaking families, I suppose).
Another option is TFS (Toronto French School). They have a campus in Mississauga for grades 1-7. It is a good school (I could be biased: I attended it). But, be warned. Its level of French is more similar to regular "french immersion" in public schools than to Etienne Brulee, College and Lycee.
So, that's my experience. Again, there could be excellent FFL (French as First Language) schools in Mississauga, but I haven't heard of any.
(I also wouldn't worry about your kids "not learning" English... my bigger worry would be them losing their French under the Anglophone culture here. Ontario is not really bilingual, you know?)