Report card comments can further explain the grades a student receives. For example, a student may be working very hard in a certain area, yet not be proficient for his grade level. A top grade would be misleading to parents, yet the student is working very hard. In cases like this, it would be helpful to check on your school's policy for grading non-proficient students. There may be an area on the report card to list the current academic level or to assign a separate grade for effort under the content area grade.
Several publishers have books available that provide report card samples.
Start thinking of comments you would like to express early. Post them on my bulletin board.