[pianoperformer]

I'm going to disagree with pretty much everyone and say you should pay attention to your grades. They are like a marker of how well you are doing, and if you need to study more, more efficiently, or just get help on certain subjects in order to fully understand them. So, if you see that you are getting a C, then obviously you need to study more, because you don't have sufficient understanding of the material.

Also, why set your goal at 80%? if you don't reach your goals, then you will get something unsatisfactory, specifically a C. Why not shoot for A's, study like you want to get A's, and if you don't then you will probably still get a B. if you only study for 80% understanding, you will probably not get much more than that.

The grade is a function of your effort. If the output is unsatisfactory, then change the input, and try again. It is totally in your control. To think it is not is only giving yourself permission to do poorly.

I also agree with Grandiouse about having uninterrupted time for studying. I would suggest, though, scheduling certain subjects for studying. For instance, maybe do 30 minutes of one subject, then switch to 30 minutes of another subject. That might help to keep you from getting too bored with studying, and keeping peak attention.

You have some time to see what works for you. You need to figure this out soon though if you are planning on going to college, because it'll be much more difficult there.