Just sharing my experience
We went in early December, and went to the Jedi Academy our first day (in case my son didn't get picked, I wanted to have another chance to go back)
We arrived about 30(?) min early, grabbed lunch from the Tomorrowland Terrace and sat on the ground right behind the line, just slightly right of center and ate our lunch. I prepped my son (who was wearing a Darth Vader shirt and not holding a sign) how to jump up and down, wave his arms, etc. Another parent was right next to us telling his kids the same thing
After lunch, my DH sat with my son (and DD, who wanted to watch, but not participate) and I snuck out to the side of the viewing area to watch and video.
Jedi Master picked mainly from the left side of the audience, but did eventually get around to the right and picked my son! There were a couple groups of 2-3 kids who got picked together, mostly 5-8 year olds I'd say, but a couple older and a couple younger. Only a couple kids had signs, and yes - they did get chosen.
My son loved it! The highlight of his trip!! He got to go up on stage and fight Darth Vader. I thought the stage "show" was really cool, with the moving platform and the Stormtroopers. Lots of fun, I'm so glad he got chosen!