I looked into things about 7 years ago when we were considering moving to Quebec with a teen aged daughter. The track of study for major subjects in high school was significantly different from what our daughter had currently been studying (subjects spiraled there while they were more separated at her school) so she would have ended up with serious gaps in knowledge. That was a large part of the reason why we chose not to move at that time.
In addition, francophone schooling for immigrants is mandatory unless you are willing to pay for private school or meet one of the few limited exemptions to qualify for public English schools. If you have any other questions I may be able to help, although I don't yet have first-hand experience, and it has been quite some time since my husband was in school there so things have probably changed a bit. We have had two more children since then, so if we move they will be schooled in French, but we would hopefully move soon so that the transition is easier.