Success in writing in exams (or even in job) requires (1) knowledge of the subject and knowledge of the language (2) how to write in organized, simple, and understandable way.
Here we will discuss item (2) only.
The best way is to ask your teacher for help. Personal tuition is also a good idea. You write some essay and get it checked by your teacher. Ask him two questions (1) What is good in my essay? (2) How specifically can I improve?
Ask the teacher for a sample essay for which he gave 100% or very high marks. Study that essay. Ask the teacher in what ways that essay is better than your essay.
Another good idea is to get together with other students. All of you take some tests. Then check answers, correct mistakes, suggest improvements among yourselves. After a few such sessions, show your answers to your teacher.
Continue this process till you achieve the desired level of mastery over writing skills.