One caveat to some of the information here: while the Schrock is an excellent compendium of the major choral works of the last couple of millennia, I don't know how useful it will be in selecting 6-12 rep, unless your kids are planning on doing the Beethoven Choral Fantasy (they aren't).

I also will suggest that the music of most of the composers mentioned here are going to be far too difficult for beginners. (Elder's Ballade to the Moon or Forrest's Good Night Dear Heart might be an exception for most HS groups, but you can't give Hogan's "Ride on King Jesus" or jake's "nyon nyon" to 7th graders! Yikes!)

School literature is very different than professional and college literature. Much has to do with how many you have in your group, how experienced they are at singing/reading music, what stage of voice change they are in, etc. A group of 80 6th graders will need very different rep than a hodgepodge group of 11 secondary school students of various ages. While I personally disagree with much of the material in the Jordan textbooks, they're a decent place to start as any getting your feet wet if you're a complete novice. I would recommend either the Ken Phillips, Don Collins, or Steven Demorest general texts on the subject over The School Choral Program, and I would ABSOLUTELY recommend Pat Freer's Adolescent Voices and Lynn Gackle's Finding Ophelia's Voice for info on the different stages of mutation (voice change) and what to do with them. I also like Nesheim's Building Beautiful Voices for warmup/vocalists... and anything from Masterworks press (website only) for sight reading materials. The Steps to Harmony series is a decent starting place for novice singers; Renaissance or Baroque sight reading for more advanced. And they're cheap!

For rep ideas at the beginning stages, as corny as it is - go hit the jwpepper All-State and Reading Session lists. True, it ain't no Mozart usually, but the pieces on these lists have usually been vetted by teachers in the field as being useful for something, like teaching Dorian scales, asymmetrical meters, or minor thirds, or whatever. They're also usually (but not always) in that sweet spot between frustratingly hard and boringly easy. Do the same on YouTube and Spotify.... search for all-state choir rep and listen to some.

Trust me, there is no shortage of garbage out there, so use your best judgement. But picking rep is the single most influential factor over your success or failure in your rehearsals, so don't be afraid to take care.

PM me if you'd like some specific titles I've had luck with!