You might have to teach yourself better study habits. It is also good to find smart students to study with. You can help each other. Also, make sure and get a good nights sleep because if you are tired you won't do as well in school. Same for making sure you eat a good breakfast and not a sugar cereal because good food will make your brain work better. Like another person mentioned, it would not hurt to talk to some of your teachers. Especially in the subjects you are having trouble with. They will know where you are weak and will be able to help guide you to strengthen things. You might also ask one of your parents or guardians to review your homework to see if it is accurate and to get extra guidance.
You should ask if you could be home schooled, if you go to a regular school you will think like everyone else, but if your home schooled you can think for yourself and in your own way, trust me I'm home schooled, and think that's part of why I'm as smart as I am.
Its a sad fact that the majority of teachers are only learning as they go, and that few are masters of their subject, masters of empathy with teenagers and masters of creative solutions. Many like to keep the flock of students under the unnecessary fears of failure, or of not recognising 7 of the 9 intelligences (only emphasise literacy and numeracy). Unless you sense a teacher who really has got life together in an impressive way, its risky to do any more than "see what they say" Like all advice from "the flock", ignore 60%, save 30% and use 10% but never let them know you threw most of it out. If you were my son I would say get out and work 5 years, earn some money, have some fun, do some short courses, pick some great mentors, make your own study and journal, you'll be $200,000 ahead of the flock and will have enjoyed life and be useful.