First of all they go through school seeing how teachers work. Then in college they have observing and student teaching to go through. The last characteristic Shulman has in the article is Community. Teachers in a school are all in a community because they all are teaching their class for the same thing, which is to teach the students knowledge they will need to posses to further their education. These six characteristics should be more than enough information to convince you that teaching is a profession.
Teaching fits as a profession under all six characteristics.
I think teaching fits these characteristics better than professional athletes.