the list by writing a letter to your child's high school. It should be done when the child enters the school because schools send their lists out to the military recruiters BEFORE junior year.
Here are links to information on the law and on opting out.
You should check what other lists (honor roll, for instance) your child may be excluding from if opting out:
http://www.commondreams.org/views05/0223-36.htm
http://www.aclu-wa.org/Issues/students/pdf/Military%20Recruiters%20QA.pdf