It's no good. The ball has to travel Inside the goal posts, so if it goes overtop it's no good.
In all my years and hundreds of games, I've seen a kick bounce off the top of the goal post once, and that happened in the third varsity game I ever worked. Not something that happens a lot. I think it might have happened in a game I worked one other time but I now work as a referee so I don't get to watch the flight of the ball.