I was in nearly the same position as you, except I was the best student in my English class.
I spent 5 years in the US, so logically my writting kind of sucked. SO I just started reading a lot of a books, there was I time where I'd read a book per day.
Basically what you should do it, it'll help more than anything, is head over to a library, get yourself a book that has also a tape recording of someone narrating it.
Then sit at home with a dictionary and listen to the tape while reading the book simultaneously (pause the tape when you have to look in the dictionary obviously).
What I found is that this increased my vocabulary, my writting skills AND my pronounciation 500%.
ALSO, as someone else mentioned, talk to the teachers. LET THEM KNOW you're from a foreign country and English "so good not so", that way they'll be much more giving & considerate when grading your papers.
Sit at home and try to write an essay in your native language. If you still have a hard time, then your problem isn't with the language -- it's your writting skills (it doesn't really matter what language it's in, it's how you collect and put your thoughts together).
