Talk to them like adults. Get buy-in and commitment from them. Then hold them accountable. If you talk to them reasonably, respectfully, they really do respond. I've had conversations with my kids where I said, "Hey, you're not going to graduate if you don't pass this class, you realize that don't you?" or "You know your Mom puts a lot of time into washing and folding your clothes and it's pretty insulting for her to come in here and see them thrown on the floor, right?"
They'll answer that they do know this (teenagers aren't stupid). So then I would ask, "So, since it's important that this not happen, and since I've tried for a long time to get you to do it and it's still not happening: what would you do if you were me, parenting you?"
You'd be amazed at how honest they will be, at least I was. They'll tell you first, "I don't know." But if you are patient and calm, and say, "I know, I don't know either. But we BOTH agree it's important that you graduate, so, we need a solution. So help me out, what would you do in my shoes?" They'll actually start confessing that they themselves don't understand their behavior and that they don't want to fail either. They'll help you devise a strategy that has buy-in on their part because they are part of the solution, but also because you respected them enough to include them in the parenting strategy. Then you hold them accountable. Brace yourself for failure, be patient, and allow them to "reset" the clock etc.--remember they are just LEARNING how to be adult even though they insist they already are--, but if the deal was they give up Play Station for a week, take it out. If they know consequences are sort of squishy because you avoid conflict after a long day at work, which I get, just know that YOU are the problem, not them. :)