Trying to get the part of Dorothy.. UGH. Help?

Just a little background on me.. I've been in every high school play since freshman year. (and seniority plays a major role in getting parts) Freshman year-I was "Misty" in the Nifty Fifties (comic relief, 12 lines) Sophomore year- Ado Annie in Oklahoma (almost 100 lines, 2 main songs) and then Junior year I was the Lead role in a comedy. Basically, I'm always the comic relief, and I don't mind that! I have an alto voice (just think ado annie.. or Kristen Chenoweth sp? style) And this year.. the directors chose The Wizard of Oz. You see, my school is small, and my grade makes up a LOT of every year's cast. Several of us get the main roles (we have a very talented class!). The only thing.. there isn't really a female comic relief part in this play.. and I want Dorothy. The only other girl in my grade that is 'competition' is a soprano... but I beat her out for the lead in the last play.. I think I have a pretty good chance.. but its just NOT a role i'm used to. Granted, I have 9 months before the play and 7 until rehearsals even start... but when I want a part I WORK for it, and under stand the characters, what they'd think, how they talk, how they sing, how they move. I'm very committed. How can I prepare for the part of Dorothy? I feel so silly being innocent and child like.. But I have to have this lead! haha, help? suggestions?