Your student is graded not by how successful the outcome of their project is, but how hard they try. My favorite thing to tell students is, "Art is NEVER wrong." Art is about freedom of expression, and I encourage students to do so. When giving a classroom a project to complete, I teach the skills necessary to carry out the project, as well as the objective I wish the students to practice, but often have the students take their artwork in the direction of their choosing. As long as the student is participating, listening, following directions, and trying their best, they will be successful.