There's no harm in submitting it early, but the electronic application form is only one portion of the whole tedious process. For me during this last cycle, I wasn't assigned a BGO until mid-November, and I wasn't able to complete an official CFA until December. Submitting it early can only help you focus on the rest of the process. Just be sure that it isn't rushed and that you are putting your best foot forward on the essay. I didn't get my application until early January because I wanted to be sure that my essay was the best that it could be. Take advantage of the fact that your high school, assumedly, offers guidance counselors as well.