What helped with my children was to sit down with them after school and do homework with them. I was going to school at the same time and would do homework at the same time.
When I was no longer going to school, I would have snacks ready for them and while they worked on their homework in the kitchen, I would make dinner. That way we were together and I could answer any questions they may have.
I will also add that I was not above bribing them. We would agree upon a goal and a prize. The goal was easy at first, say individual tests, and worked up to harder goals, like 1 letter grade improvement in one class. The prizes will depend upon the goal.
Work with your 13 year old to do research and projects that will make it interesting to them.
Good luck, it's not easy to keep them interested, but if you can capture their imagination they will work really hard. Remember...even if they are reading comic books...they are reading!