Hello, sorry this is late. I just noticed it was here!
My son is the same way. He started out first quarter with a 4.0!!! Now we are into 3rd quarter and he is getting a "C" in one class and mostly "B's" in the others!! I think there are 3 things happening, at least for him.
In the first part of the year, you are getting used to school again. Everything is new, and you want to keep on top of it. By the middle of the year, he gets comfortable, quits working as hard.
The second thing is that .....he isn't all that organized and by the middle of the year, he has so much stuff in his folder he loses focus on what he has and still needs to do.
Third, it is spring. He does basketball, spring soccer, and there is the "spring fever" factor also.
I try really hard to make him write things down on a calendar, and clean out his notebook every week or so. And the hardest of all is teaching him to manage his time. I have trouble with that, so not sure how to teach him! lol lol