Haha, school. I guess I should go back, I've done nothing but play Final Fantasy 10 all holidays! (I'm serious =/)
Anyway kiddo, the best way to get your marks up is to study hard. Find a way of studying that works for you. If you like doing hands-on stuff, I suggest you find a way to make the subjects you're bad at a bit "physical", so you can learn it hands on.
Keep a journal of when your assignments are due, study 2 weeks before the assignment, and a week before start getting notes of what you remember (if its an essay) or start working on the actual assignment.
If you plan ahead, it does actually help out. Though, 9th grade is piss easy, I'm in it at the moment, and I was hoping for something just a tad harder than 6th grade, you know?