Mass I, VIII, IX, XI, and XVII or XVIII.

Some of the seasonal Communion propers (Qui manducat, etc).

Requiem Mass propers.

Some principles of semiology and at least some basic experience with the Triplex.

It depends, I suppose, on how you see the focus of music education in the diocese. If you are focusing solely on sacred, mass-appropriate music, I would make the primary requirement be the entire Jubilate Deo booklet from Pope Paul VI. The next step, if you want to go further, would be some of the propers, the most common sequences and some additional chant mass settings. Here you could add the requiem mass propers and such. Then you would look into polyphonic mass settings and motets. Ultimately the final few steps, I think, would depend on the numbers you have in your programs.

Ultimately, though, I think that finding music that teaches the skills you want the students to learn is more important than a set list of repertoire they must all know. I would rather see a list of the skills and abilities you want to students to achieve and learn instead.

This doesn't exactly go along with the question, but teaching them how to navigate through the church's liturgical books would also be a very helpful skill. If they know enough about the liturgical year and enough basics of Latin to be able to find where they should flip to in the Graduale Romanum and Simplex without being need to be told "the page number" they will be instant leaders of the future.

Some of the answers above seem to be oriented toward a Catholic music appreciation program, (B minor mass?) more than a practical course to produce Catholic musicians. I also question the repertoire-based bias of the question. Here's what they need to know by the time they leave high school: the rudiments of voice production, and the ability to reliably sightread chant and any 16th-century polyphony (they should also have experience in reading earlier and later music, but it's best to have them master one musical language). I like the idea of starting with chant, since there is so much there that is foundational to later music, and limitations in the language that make it easier to master from a literacy point of view. After that, what one wants in an ideal world is a set of graded polyphonic music readers, but I don't know that such a thing exists. The Rhau Bicinia of 1545 were designed for such a purpose among Lutherans; there are a number of Latin-text pieces in there that could be useful, many in canon.

This approach doesn't necessarily negate the Great Books approach, but subsumes a canonical repertoire to a higher goal. If you're looking for titles, probably the "Arcadelt" Ave Maria, Byrd and Mozart Ave Verums (maybe Faure too) etc. ... basically the chestnuts, as well as representative samples of specific genres.

I am reminded that 10-12 year olds are singing the treble lines in some of the finest choirs in the world. If you teach young people quality music of any historic period, I firmly believe that they will accept the challenge and rise to the occasion. Teach them to be musical and well-read in the great treasury of choral music.

I wonder how often many of us teach our choirs a longer-than-5' piece?

I wonder how many of us have liturgies that can contain a longer-than-5' piece? Not that the liturgy needs to be the final arbiter of what music to learn, but first things first. And how many public schools do longer than 5' pieces, or pieces with orchestra? That doesn't need to be the standard either; we can do better. But there may be good reasons that such things aren't done.

It begs mention that, to my knowledge, in all likelihood diocesan departments of education basically absorb and adapt (very little) public state education curricula, none of which is deigned to have any Catholic emphasis. I've culled through ours out west here, and it is so broad of scope, and as our Canadian friend posits, so lofty, expansive and comprehensive, not even those of us with post-baccalaureate degrees could meet the criteria with the limited classroom time alotted in K-8.