Grading art is not like grading any other subject. It is probably one of the most difficult parts of my job. The first thing I do is give very structured guidelines for an assignment. That is the foundation for the grade. If you follow the directions and work to the best of YOUR ability, that is all I can ask for in class.
Contrary to what a lot of students think, we as art teachers, don't line up student work and give the "really good" projects the "A's", next in line the "B", etc. Rather art is something that is graded not by talented but rather by trying. If you work and you work hard, fulfilling all of the required aspects of the assignment, then you will do great! On our grade rubric for each assignment, there is always a spot that asks if the student has spent time wisely, etc. That counts just as much point wise as parts of the technical assignment.
A lot of students are afraid to take art because they don't feel that they are "good" enough for the class. In my eyes, those students are some of the best I get. They are ready to learn and try new things. So when you think about taking art or taking an art class that isn't in your comfort level, go for it! If you work hard, you will do great!