Riffing off an earlier post about curating repertoire. I'm particularly interested in input from the music education world (which I'm not from). And this is focused on polyphony, not chant. One could I suppose construct a grading system for chant in which the Chant Ave Verum Corpus is Grade 1 and Jubilate Deo Universa Terra or the tract Qui Habitat is Grade 5. I'm not sure what the point would be, since one doesn't choose chant by level of difficulty but by liturgical day. And it's not clear to me how chant as a whole fits into an overall system.

There are several grading systems out there on the Net. They are all short on specifics. A rough estimate:
https://alansimmonsmusic.com/about-us/difficulty-levels-explained/
• Very Easy/1
Works in up to 4 parts containing
simple rhythms and harmonies, often with supportive
keyboard accompaniments. Ideal for choirs with limited reading ability.

•• Easy/2
Works in up to 4 parts, mostly with easy rhythms and
harmonies and helpful keyboard accompaniments.

••• Capable/3
Still in up to 4 parts, but containing some challenging
passages, with or without accompaniment.
Require some basic reading skills.

•••• Skilled/4
Works in any scoring, with or without accompaniment,
containing some challenging passages. Require solid reading skills.

••••• Advanced/5
Aimed at selective and professional choirs,
these works are both technically and musically challenging.

Now, in developing specifics, I would suggest a principle: that a grade for a secular choir might not match a grade for a secular/educational choir, just because of the differences in personnel and training. Unlike a high school or even collegiate choir, a church choir is usually made up of mature (often overmature ) singers. And their training and routines are somewhat different. I would expect a piece that is not in the cultural vernacular language to be graded higher. But Latin is part of the cultural vernacular of the Church. (I use 'cultural vernacular' because a piece in Vietnamese aimed at Vietnamese masses will presumably be sung by people who know how to sing Vietnamese.) Vocal ranges might also differ. Tenors and sopranos above F and altos/basses below A suggest a slightly higher grade. Likewise descending 4ths and 5ths, (in any part but bass) or large intervals. Amount of voice crossing and departure from homophony will matter in the lower grades.

Where is Palestrina? 3.5, or 4? Whatever it is, Ockeghem is higher, because of the rhythmic language More than 4 parts means level 4, but fewer parts doesn't necessarily mean a lower grade (cf. Dalitz Tribus vocibus vs. Byrd in 3). Only a very few elite Catholic choirs will be doing Grade 5 music on a regular basis, if for no other reason than rehearsal time.

This is me picking at a possibility. Is there any need for this at all? Am I measuring manure with a micrometer?