Projection thinking is the key
One of the biggest issues with getting teens to care about school, is that they often dont give a toss about the future. The part of their brain that deals with things like consequences and repercussions is not fully developed. Literally.
So the cruel irony is that the time your teen is at high school is one of the most important times for them to be thinking about the future, but many arent able to.
This means its up to YOU to help your teen think about their future. Not many teens do this without encouragement.
A huge motivator for me at school was my parents pointing out how un fun it would be to spend my days working behind the check out counter at the supermarket because thats what would be waiting for me if I didnt do well at school. Whether thats what would have happened or not is up for debate, but the point is it was a great motivator. Maybe something like this would motivate your teen too?
If I had to some up this post with one word, it would be positivity. Thats what has the best chance of getting your teen to see the importance of putting in the effort at high school.