Through the use of very specific criteria and expectations, you can develop a grading system that is challenging, understandable, and fair to all your students. Through the use of various assessment instruments and techniques, you will have ample evidence of achievement outcomes. I always recommend a book I have used in the past few years.
Many art teachers know of it and use it.
In fact about six months after purchasing this book for myself, our county art ed. You can order it from them or directly from Davis Publications. They have a picture on their site too. However, they use frames for their catalog and it won't give me the exact address for the page of the catalog it is on. You can order it online from Davis.