Well, you do know why you're here, right? You're here to discover the secrets of doing better at school. :P Anyways, let's get to the point...
Attendance DO NOT SKIP SCHOOL! If you want to do well in school, you must stay in school as much as possible (unless you're really, really sick) because for example, if a test was tomorrow and the teacher was telling you what to study and you weren't there, you'd do much worse, right? You'll miss lots and when you get back, you'll be behind in work and sometimes it's a pretty deep hole to crawl out of.
If you're sick or you can not go to school, arrange for you, your parents or your friends to pick up your homework so you don't fall behind!
This one is a big one too - after all, if you don't pay attention, how can you learn? Try to avoid getting distracted during class and if your friends want to talk to you, ask them to wait until recess or break. If possible, try to take notes in class - it'll help you when you study for tests.
You must proofread your work! I for one have lost more than a few marks on Math tests because I forgot to proofread them. If it's a writing assignment and you're doing it on the computer, spell check and grammar check isn't going to do the trick - you yourself must actually read the thing you're writing because sometimes the computer doesn't pick stuff up.
Try to avoid procrastination at all costs - you'll always do badly when your procrastinate. For you readers who don't know what procrastination is, it's basically doing stuff at the last minute (or last second). Try to write down stuff you need to do in your agenda or on sticky notes so you don't forget.
Remember in Legally Blonde 1 when Elle Woods hands in perfumed fancy pink paper as her assignment? In real life, that isn't going to get you anywhere. When doing work on the computer, always use readable fonts (like Verdana, Arial, etc.) and try to keep the font colour black. Oh, avoid using coloured paper - stick with good old white paper. Why? Professors and teachers hate it and really have a hard time marking assignments with unreadable fonts and coloured text - you'll end up with an even worse mark when you think you'll get a better one.
Attitude and fighting
Avoid giving anyone attitude, even when you think they deserve it. After all, treat others the way you want to be treated. It can also give you suspensions, which is not good at all.
If someone says something to offend you or is trying to make a fight with you, ignore them and go to a teacher ASAP. Therefore, they get in trouble and not you. If you decide to go physical with them, you both get in trouble, but if you ignore them and talk with the teacher, you'll be fine and the other people won't be.
Always give your best effort on assignments and tests. Some teachers give you half mark on questions if you get close to the right answer or if you try. Who knows? You might just get lucky...
Organization and hand assignments on time
Try to keep your workspace clear and free of clutter (your desk, in layman's terms). Clean your desk out every week or so and avoid stuffing papers into it - throw stuff away when you don't need it.
Remember to hand your assignments in on time! Most teachers take marks off your assignments/projects if you hand them in late. And remember to never procrastinate.
Just follow the tips above and enjoy your improved report card! (Oh, if you do get a good report card, tell your teachers, parents and friends about this page :D)