If you want to play jazz. If you want to get better at jazz. If you want to spend a day on a bus with a bunch of cool people and eat at Panera and just be cool. If you like music. Do it.
The audition materials are here. The MMEA files are here, like the audition tracks and sample tracks.
Freshman are strongly encouraged to try it out. It's unlikely you'll get in as a freshman but it's really important to get the experience when you are young! DO IT!
From the handbook: "Classical/Jazz All State:Every year, MDIHS sends dozens of kids to participate in all star jazz bands, choirs, combos, orchestras and concert bands comprised of the top musicians from around the state, directed by all star musicians and directors from across the country. In order to participate in these ensembles, students must audition. We strongly encourage you to prepare an audition and participate! Regardless of whether you make it in or not, anyone who puts together an audition and gives it a shot grows exponentially as a result. Many folks who audition and don’t make it in as freshmen, make it in with above average scores as sophomores. If you are serious about participating in these ensembles (which you totally should!), you should start early- as freshmen! It is a great honor to be apart of the process. MDIHS music typically has among the highest representation in the state at these functions! If you are interested, chat with us. We will do whatever we can to help prepare you individually for these events. Jazz Improv and Voice Class are designed to help you prepare stronger auditions too! Get involved!"
You want to get good? You should play along to this AT MINIMUM twice a day. Don't stop- just go, all the way through. Want to get real good? Do it more. Even when you don't know what you're doing- just DOING it is HUGE. Do it 5 times a day. 20 times a day. ALSO, practice to DRONES