For some of you the word "grammar" sounds like "pain" or "boredom". But it CAN be simple if you learn constantly. You should learn from class 5 to class 10. Regularly! And of course: You have to revise the things you learned in previous schoolyears.
Moreover: Analyse your tests. If you reached less than 50% of the maximum credit, make sure that you know which kind of mistakes there are in your test. In general there are 3 basic errors that can be found in tests:
But most importantly: Ask your teacher what your problem is. Teachers are there to HELP you, believe it or not