In an ideal world, school leaders would have a few years of experience in the classroom before they take a building role. I feel like principals need to have, at least, a few years of experience in the classroom to truly understand students and learning. The transition from teacher to a leader should be one that comes naturally.
As a principal, It made me nervous when pre-service teachers said they wanted to be a principal...before they ever experienced being a teacher. I think it's ok to have the goal, but focusing on teaching and learning for a few years should be the main priority. However, through social networking formats like Facebook and Twitter it's pretty clear that many principals do not have an overabundance of experience in the classroom. Some are jumping into the role after a year or two.
In a previous blog where I wrote that being visible isn't enough when it comes to school leadership, Dr. Spector wrote a comment that helped me realize that the road to leadership is sometimes paved in sports, and not necessarily experience.