In school I am totally good at writing but whenever they set a prompt in front of my face my synonym knowledge flutters away from me. If I'm exceptional at writing except for prompts will that hurt my chances in getting in a high Literature class this year?

The key to getting better at anything is practice. Go over and over what you need to get better at concerning writing. Work on this with a friend, your parents or the teacher. Most teachers are willing to work on things with students outside of the regular class time.

Not all people who are great writers necessarily think really quickly. Some take a great deal of time to think out about what they are going to put into their writing.

You also might want to consider purchasing the book that is advertised on the following link: 350 Fabulous Writing Prompts (Grades 4-8)

If a teacher finds your writing to be good and that the potential for you to be exceptional at it exists because it is in your heart to write, I am sure that he or she will do everything they can in order for you to be in those classes that would help you to achieve your goal in writing.