This letter outlines the general expectations of the class to the students and their parents. The students' first graded assignment in my class is to have their parent/guardian read and sign this document to signify their understanding that it is a college-level, rigorous biology course. Additionally, the student is expected to sign it as well.
This has helped me several times when students and/or parents complain that the course is too difficult for a "high school class." I simply show the signed contract to the student/parent/school administration, proving that the student and parent agreed to the terms of the course and completely understood that the work is expected to be college-level.