My situation is similar to Cindy's. I am in an MAE program for Special and Elementary Ed. Right now I'm teaching 6th grade reading half days for summer school.
Each school in the district (one primary, elementary, middle, and high) each set their own dress code, but most are pretty similar. Theoretically jeans are out, but dark washes seem to be acceptable. No flip flops unless they are nice ones, no t-shirts. Professional attire is required. The middle schools where I'm at now is slightly more strict, the principal is new and is trying to instill discipline. I was a little mad because I bought a really nice dressy looking pair of flip flops and then found at the faculty meeting that even those were considered unacceptable.
The sad thing is the student teachers look more professional than many of the teachers. Our dress code set for us by our professors is even more strict (no jeans at all). My wardrobe this summer has been a skirt or nice capris, my Isotoner heeled sandals (the kind with one thick band of leather across the toes), and a rotating batch of polos and nice blouses.
I believe that the way you dress does in deed set the tone. I expect the students to treat me with the respect. After 2 1/2 years on my master's, they better respect me by golly!
My problem is, at 4'10" many of my students are my height or even taller. I spent a month in the elementary school last year and even in my professional dress the cafeteria ladies kept trying to sell me the kids' lunches- they thought I was a student! I think that for me, it is necessary to set myself apart from the students in the way I dress, otherwise no one would know who the teacher is! (I frequently get mistaken for a 12 year old, aside from being short I also have a bit of a youthful face.)