hey,
i did somthing very similar only recently but with 40 kids and know drums, and to 2 track (old skool)
i would consider putting a duvet or isolater of some kind in front of the drums.
you will most likely get a lot of spill form the snare and cymbals..perhaps ask the drummer not to use stick and use reeds or similar...
with know audience i would place the choir and band facing each other...perhaps using the floor space in front of the stage if space allows..
i agree with placing the band around the piano. make sure that the drummer has a good view the director as he/she is the click track...
kids gets tired and bored fast so make it as funny and as enjoyable as possible...tell them that you will let them hear it at the end...
dont let them have to much sugar (that probably goes for all musicians - especially the drummer
with the piano i would consider recording in stereo if trks and equp allow..but keep in mind that with so many instruments mono would be fine...so make sure that if using two mics on the piano that one has the overall best sound
ask any lead singers to step out a foot or two from the choir and but a condenser about 3ft in front (with pop shield)
mic choice: i really like two km184 on the choir (x/y) / u87 on soloist / 414 (*2 ?) on piano / sm58/57 on amp / di and condenser on bass / drums...i really depends on how there played, and how the finished product is meant to sound.....i like the minimal approach..
thats my 2cents...have a good one....