This will sound pretty nerdy, but i found getting most of my homework done during lunch breaks, recesses etc, is extremely useful. It frees up afternoon and evenings to do extra work/reading. Also, if you do your homework pretty soon after you've had the lesson, it means everything is still fresh and if you're stuck, asking a teacher for help is much easier. One of the most important things i can suggest is ask for help. Talk to teachers, go to subject clubs/drop ins if your school has them, just make sure you consolidate your knowledge - after all, teachers get paid to teach and helping you should be their priority