Nice choices! I like them all. Hadn't heard of the Shackleton one, we do Antarctica in year 5/6 in NSW so it'd work really well! I've done Nigel Westlake's Antarctica suite before with 5/6 and they really enjoyed it. Guess this would be somewhat similar!
Dona Nobis Pacem is also lovely (in the style of la-te-doh-re). Three part round and as many ostinati parts as your choir can handle.
What? No Hallelujah? Get with the times. Everybody's doing it. What's a choir without a bit of Leonard Cohen.