I don't know if this will be of any use but in the UK there are music exams from Grade 1 (easy) to grade 8 (hard). They are run by the Associated Board of the Royal Schools of Music (ABRSM). If you go to their web site http://www.abrsm.ac.uk/ it might be worth looking at what they suggest as pieces for the different grades/levels for Singers.
I don't know what you have in the US but I've spent half my life doing these exams on various instruments and I always found that the choice of pieces were well chosen to reflect the musicians abilities at all levels.
Kirsty
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