For PUBLIC schools: If you are an immigrant (through QSW or other provincial or federal programs) you MUST send your kid to French school, unless the kid has already spent sometime in English medium public schools in other provinces of Canada (e.g. Immigrant landed in BC, sent the kid to English medium primary school in Vancouver, then moved to QC and the kid can go to English medium school in Montreal). If you are a temporary resident, your kid can be temporarily exempted from French school (can go to English school), but that permit expires after you become PR and the kid has to switch to French school.
If EITHER of the kid's parents have had primary or secondary education in English in CANADA, the kid will be exempted and can go to English school throughout the whole primary and secondary education.
Of course, if you choose to send your kid to private school, you are free to send him/her to either English or French medium schools.