If your ultimate goal is to march in the Bluecoats, you should audition there this fall. Even if you know you're not at all ready, you'll learn what it takes and what they expect. This will help you prepare for the next year. Here's how I would suggest you approach it:
A) Work your butt of to be as ready as you can be.
B) Learn as much as you can about the organization and their audition requirements. Most corps publish a packet of information and audition materials.
C) Know going in that you may not be ready and have a backup plan to go somewhere else. Knowing that your end goal is pretty high, I would suggest you aim high and take a spot in the best corps that will have you. That will give you the best preparation to move up later. You may have to audition at several corps. (Most offer more than one audition camp.)
You can march the season if you're 22nd birthday is after June 1. (So those with birthdays between June 1 and Finals night can march at the age of 22.)