It was simply that the offenders would "get in trouble" with the teachers.
Repeated offenses would be reflected in their marks at end of term; that
sort of thing. No one was caned ;-) . But you can imagine that a class of
children, ever more firmly remonstrated to use only their French, will begin
doing so; the teacher's authority still holds significant power for them. If
this approach were begun with a bunch of recalcitrant 15-year-olds, it
wouldn't work.

I visited both children's classes a few times -- they did indeed constrain
themselves to French in their in-class peer discussions, and even to some
extent between classes. Not at recess so much, though! Also, remember that
by grade 2 or 3, it seems quite natural to these kids to use French in their
classrooms and with their teachers. When I would go to meet-the-teacher
nights, my kids would speak to their teachers in French and turn to me and
address me in English (even if I was talking to the teacher in French).
Which I got a kick out of, but the fact is that code-switching like this
goes on all the time all over the world -- it was only unusual to me because
I hadn't been around it much.