Study for so long (30-45 minutes) then take a planned 5-10 minute break to get drink, stretch, use the bathroom. (come on Ask)
Write down important things. People tend to remember things that have written down.
Play classical music softly in the background. I know you are probably thinking I am nuts but there have been several studies that show it helps and I know people who do this. Some have taken this habit with them into the career as well when doing spread sheets and such.
Number one tip is to always stay focused. I know it might seem really hard, especially with all the boring subjects but it's a great tip. Try taking notes in class when your teacher is talking. If your teacher doesn't give you a study guide remember to make one yourself. And also if you are a colorful person (like me) highlighting important things in your notes or book is a great tip. I hope this helps. Good Luck :)
1. find a quiet spot; after school such as library or lunchroom; 2. have small snack before or middle of homework break; 3. have friend or family member help you by giving quiz/memorizing answers. good luck!