I try not to put any emphasis on grades with DD9.
She is high anxiety, and if I push too hard she will literally break down. She was averaging 42 in Math. I worked with the teacher and we discovered she only does bad with timed quizzes. So we have all been working with her and are putting a 504 in place for test anxiety. She is now averaging a 97 
So I think if you just watch what your child gets for grades and work with the teacher to see how they can get help if they need it, no punishing needed.
If they dont hand in homework or are lazy, then they dont deserve any electronics or distractions until they correct that.