Customize your registration. Ask for customized registration rather than computerized registration. This will let you carefully choose the best periods to take your most difficult subjects. Take them at the times of day when your energy is highest. Work hard to develop a good relationship with your teachers.
Teachers usually work hard to help students who seem involved and motivated.
Try to get them to help you solve your problems. Don't wait until there is a problem before you talk to your teacher about your add. Let your teacher know you are trying. Attend summer school. Consider taking a really tough course in summer school, when you can concentrate on it without all of the competing courses and activities of the school year.