Although I didn't take the obvious route to becoming a high school art
teacher (I have a BS in Design and my credential through an extension
program). I feel that I missed out collaborating with peers within an arts
education program to help me get started. Without the wonderful mentoring I
received from fellow collegues in the art departments I wouldn't have
survived. You will find that most K-12 schools will provided you with
curriculum and lessons used in the past. These will be a stepping stone for
developing your own lessons.
One thing I didn't have in my studies is the Visual and Performing Arts
Standards (maybe you covered them in your studies). You will find that most
curriculum is based around these standards. And you will find it very hard to
include all of the standards into your instruction time. I feel that this is
the most challenging aspect of teaching art in K-12.