You can get better grades by first of all attending all your classes. This will allow you to get all the classroom materials and earn attendance points. Then, remove any distractions while studying. Finish all your assignments on time and study in short intervals of 30 to 60 minutes. Follow good rules of writing and use time wisely.
Pay attention in class. ATTEND class. Write down notes about the lesson in class. Study after school. Time management. Find a tutor, friend, family, whatever. Get rid of distractions(TV, internet). Read educational books!