A very good way to improve your own writing is to read other people’s. If you read plenty of good English fiction, you will learn how skillful writers create interesting and believable characters, develop plot, explore a theme, and so on. If you read effective non-fiction you will learn how good writers present information in an interesting and organized way; you will also learn how they use language to instruct, move or persuade.
Reading lots of books can also help your writing in a more general way. This is because all the time you are reading, you are learning different aspects of English. For example, you are learning new words and expressions to use in your own writing, and at the same time you are improving your ability to spell correctly. You are also learning English grammar; particularly if you read with an awareness of language and how writers express themselves.
You can improve your writing, like any other skill, by practising it. Of course, you have to do many different kinds of writing for your subject teachers, but you could try to write something extra at least once or twice a week. Here are some ideas:
The writing process is the name for the series of steps that you should follow when doing longer pieces of writing for your subject teachers; for example, an analysis of the causes of World War 1 for your history teacher. There is a detailed explanation of the writing process elsewhere on this site.