Don't sweat it too much man.
I've been in your position before and know what it feels like.. and what its like being under pressure from your parents to do well in school sometimes. I have also felt in the past that sometimes my parents forget how difficult certain things in school can be, especially considering they are so used to be younger brother being a straight A student his entire life, they expect the same things from ALL of us.
If you did poorly and your teacher calls your parents, sit down and talk with them. Apologize for your mark, but explain to them that you really did try hard and you did not do so well and that you are disappointed in your mark as well. If they think you spend too much time on the computer (I think all parents think this of their children), then try to dedicate a little extra time everyday to studying and/or doing your school work so that your parents can see with their own eyes
that you are making an effort.
To summarize, it happens man. So there is no sense beating yourself up about it. Just pick yourself up, brush off the dust, and try even harder next time