Improving your English is done by really paying attention in class, boring I know, but its helps. Also if you don't understand something you could always ask someone, yes even the teacher, and trust me they will not think your stupid for asking. Reading does help because you see the way words are used and it can improve your vocabulary. I am a writer and being one often helps when I read. When I was younger I was able to right creative stories, not from reading, not from school, but rather from my own imagination.
The one thing though is when you right you have to be yourself and put yourself in both the story and the writing. Many people find English to be boring, I did too, but after you learn you will be able to use the techniques to write. So just try your hardest. Read a lot more, if you so please, and make writing personal. Having a keen eye for English structure of words also helps you improve your grades. I guess it comes easy to me, math on the other hand was not my forte. :~) Also there is many websites that can help you with English, just search under such terms as: english help, high school english, and english homework help. Good luck!