In a concert band, the loudest instruments are brass (especially trumpets) and percussion. You want those as far from the choir as possible. Since the woodwinds (flutes, clarinets, saxes) are generally softer, I'd put the choir next to them. That would mean the choir is on one side of the
stage and band on the other. That will also help the choir sing because they can hear themselves better than if they were split in two. Hopefully, you have a cooperative and understanding director. Diplomacy and explaining the reasons may help.
I would use any kind of
cardioid mic on
boom stands. A small
diaphragm condenser mic would be the choice, but whatever you have a few of will work, even SM58s. It would be best if all are the same model. More
spacing between mics is better than having them too close together. Spread them every 6-9 feet across the choir. It's tempting to put many mics on a big choir, but 4-5 is all you want.
If you want to get into the theory, the
spacing should use the 3:1 rule to avoid comb filtering from multiple mics picking some of the same voices. For every 1
unit of distance from the source to the mic, the next nearest mic should be a minimum of 3 units away. The number of open mics rule (NOM) also comes into
play. That says that as we increase the number of open mics, the less
gain before
feedback we can get. It drops by 3 dB every time the number of mics doubles.
On your
mixer, apply high-pass filters on each mic. If adjustable, 100 Hz would be good to reduce
pick up of
bass energy from the band.