I have been preparing for SAT for 2 months now. I am consistently getting 800 math and 370-390 on reading sections of official practice tests but am struggling to even get a 350 on the writing section. How do I improve my writing section score?
Adding on to the other answers (which give good advice), I'd recommend you read as much as you can! Read books you love, read interesting articles (personally, I'd stay away from unedited writing like fanfic or internet posts, as they often are not grammatically correct), anything edited and high school level or above that you can get your hands on. Reading helps you learn the "language" of good/grammatical writing - you start to become fluent in where the comma should be, because you've read it written the right way a thousand times.
I would also recommend that you write as much as you can! Answer short story prompts, draft letters, anything relatively lengthy (200+ words IMO). Then, have a friend/teacher/parent review your work and point out mistakes. This will also help you "learn the language" without prep books or Khan Academy, and you'll have the added benefit of more permanent improved writing skills.
Hope this helps!
I removed my answer after receiving the last comment.
To keep this community safe and supportive: