The causation is clear. If you have free time, you spend it. There might be an indirect causation which is homework vs games, but I suspect that students with

The causation is clear. If you have free time, you spend it. There might be an indirect causation which is homework vs games, but I suspect that students with less encouragement from their parents to do their homework spend the time on games. So forbid homework and make the schools take responsibility for actually learning their students something.