First thing to do is to try and get your teen on board
As youre probably all too aware, nagging is not an effective long term solution for motivating your teen.
If you want your teen to do well at school, theyre going to have to get on board with the idea.
Try and have a chat with your teen about adding regular homework/study time to their weekly routine. Talk about the future and the importance of doing well at school. The key will be to keep the conversation on an even playing field. This should NOT be a lecture. Their input should be just as if not more important than yours. Your teens grades will benefit the most from them being self motivated.