I can't speak for Canada but in America your HS grades don't really matter when it comes to flight training. It's just not an issue. Still, getting into college is important so you have to have somewhat decent grades. I did well in math and physics but was an overall 3.0 in high school. However, to say you NEED to be good at math and physics to fly a plane is wrong. You just have be able to understand the concepts of flight. It's not rocket science, though some like to take it to that extreme.

I sometimes think guidance counselors in high school just assume you need to be a math whiz to fly. They just don't know any better but it isn't so.

Having good study habits and being able to do well in a fast paced ground school is important but I wouldn't sweat a low grade here and there.