For looking at the various music lists (classifications), I suggest you go to the JW Pepper site at: www.jwpepper.com/catalog/welcome.jsp
You can check out the lists of various states or regions of the country for comparison, clarification or research. SCSBOA does recognize the UIL (Texas) List (classification) for SCSBOA Band & Orchestra Festivals as well as the SCSBOA lists.
A higher rating (class) does not always mean a tune that has more notes, difficult rhythms, difficult key signatures, or extended range of the instruments needed to perform.
Irish Tune From County Derry by Grainger/Rogers has long been classified a class A piece for SCSBOA Festival. Does the piece include rhythms, meters or key signatures that are difficult? Absolutely not... However, the scoring (use of voices), exposure to individual lines/voices and the range of some voices including upper clarinets and 1st horn deem the tune to be of a level of an "A" group to perform well. Also, the use of suspended chords and color tones calls for intonation of the highest level.
Many years of careful evaluation by professional, military, university, college, and school music conductors through out the United States has "placed" most band and orchestra literature into various "classes" for lack of a better term.
Young music teachers with less experience use their state or region music lists as a guide for music selection based on the ability and/or needs of their ensembles. Music lists are looked upon by most as a "guide" for directors and not always the final "end all or be all" for everyone.
Most music teachers choose to play literature of musical worth and of long lasting intrensic value for their students as well as themselves. I know of no music teacher that likes to rehearse poorly written music for weeks or even days on end... it wears on our ears very quickly. (various arrangers/composers come to mind at this time...)
Hope this helps stir some thought and interest...