I don't think you win this power struggle. If getting good grades comes down to your demands, she won't be invested and you set her up to sabotage herself, either now or later. I've seen it happen. "All my mom cares about are my grades, its how she measures my worth, so fuck Mom I'm going to tank." I'm not saying this is what you think, but ultimately you cannot make her get better grades. You can punish her as much as you like, but she holds all the cards here. And push her, she could push back, and she will win.
I would first find out why her grades have gone down. Is the material too challenging for her? Then maybe get a tutor to help her get through exams. Is she distracted by electronics? Then create a specific structure for when she can use them, not as punishment but as support. Is it possible she's depressed? A drop in grades can be a sign of that, in which case punishing her will only make things worse.
Ultimately it is developmentally very helpful for kids to get at least one B. The kids who get straight A's never learn to recover from "defeat". They have no resilience. Of course, if her grades are tanking all around, I would go back to what I just wrote and try to find out why.