Our process with the kids:
Studying is not negotible. What time to do it is. DS usually likes a break after school--he rides his bike almost 2 miles home, and the weather can be yucky (muggy, rainy, occasionally cold, usually hot). He comes home, gets a snack and water, and can play until dinner. (Unless he has chores, then those get done before he runs to play). After dinner, he does his homework/studying. DH is usually on the machine so he has time to help DS.
Tuesdays are different, due to Scouts--he can mess around for an hour, then does his homework in the car on the way to dinner/scouts.
Our goal is to have them do their best, in anything they do. If our kids can honestly only make a C or a D, and they are trying their best, we are happy with that. But they have to be responsible and try to get things right.
My DD2, no matter how hard she tried or any amount of tutoring, could only pass geometry with a D. She got algebra with a lot of hard work, but geometry was not clicking. Science and English were her strong subjects, math was hard for her.
One year, DD2 had a teacher in math that was horrible--he would assign them pages in the book and then spend the rest of the class period on his computer. No help, no explanations.

I think that is when she started to struggle. (We worked with her on the subject, but she wasn't getting the help she needed at school--and failed several exams because she didn't do the process the way it was done in the book; she was doing it the way we taught her that made sense to her).
DS is like DH--they think math in a way that is completely foreign to me. For example, 281-159, I would carry over from the tens, do the ones, then tens then hundreds, to make 122. DH goes 159 is close to 160 so 281-160 is 121, so that's our start point, now add the one you dropped in to make it 160....ARGH!!!!
Each of my kids learn differently. DD2 needs to be taught step by step, verbally and visually, and needs a bit of help the first few times through. DD1 needed to be taught verbally or visually, but could go on from there with minimal effort. DS is more like DD1.