Is your focus on the right thing
The next thing is to make sure that the focus of your encouragement is on your teens effort, and NOT their grades.
Of course grades are important, but if your teen is completely unmotivated about school, then weve got to get a few things sorted first before getting straight As becomes the goal.
If your teen hates school, encouraging them to get good grades is pointless. They may not even believe its even possible at this stage.