I agree whole-heartedly with "I Only Sell". Think of the next couple years as a temporary arrangement. Your objective is to get good grades and to get average/above average grades. You DO need to build a good base, or else your tower is going to collapse as you built on top of it. I know this may be a generic response, but if you are the kind of student that doesn't understand concepts very well the first time, you will need to study. Studying sounds like such a shitty word, because it often involves shitty work. But if you can make studying, dare I say, "fun", or at least manageable, then it'll work out for you in the end. I typically listen to radio or internet radio that specialize in remixes, music has been proven to stimulate one's mind. Anyways, from where I reside, if you don't have 75+, you're FUCKED. But, if your grades are below this, no need to fret. The best thing you can do at the moment of reading this post is telling yourself that you will start doing better, and commit to it. Less time on the computer/xbox/tv/whatever. As for your parents/teachers, don't let OTHER PEOPLE influence YOUR FUTURE. Many successful people have come from very rough situations. And when you get home, planify a reasonable time to do your homework, and COMMIT. When you see that clock strike 5pm/6pm, don't THINK. Just open that goddamn book and get into it.
Recap :
Don't fail ANY classes.
Aim for 80+, you will get somewhere near that if you commit to this goal.
Don't let others dictate your future.
Do your homework. There will be many forms of homework outside of schooling.
Hope I helped. GL and HF.