There are various examining boards which offer specifically graded collections of pieces across various styles from which a student learns a set number of pieces to present (along with a set of technical scales and sight reading, aural skills and general knowledge) for grading by an examiner.  Many students wish to learn without the pressure of exams, but want to have an idea of what grade they are up to.  Studying pieces from any exam syllabus is a great way to assess your readiness to try a harder level piece in an area of strength without studying a whole set of pieces at a harder level. The AMEB Piano For Leisure Series has been enjoyed by many of my students for it's variety.