How can I improve my English grade?
This is a process; it's a marathon, not a sprint.
First: Read, read and read. But challenge yourself. Your vocabulary will not improve if you read the same types of books over and over.
Second: Read quality journalism, such as The Globe and Mail; New York Times;
Third: Listen to your teachers' suggestions and apply them to your writing. What's the point of writing the same way on every assignment?