The only way I can think you could do that, would be if you recorded one person at a time. Or if you did the whole group of people you would have to get a headphone for each person. And a headphone monitorings system (with enough outputs for all the headphones). $$$$$$$$ :x
If you record them all at once....put a mic in front of each person (an inch away or so) for each singer. Then get 1 or 2 mic's and put them outfront of the group to capture the "group" sound. (Say about 5 feet away, and 5 feet off the floor, and 5 feet apart) It should work out nice.
But you have to decide if you want to spend $$$ on headphones and such or if you are gonna track 1 person at a time.
You could try to position the speaker cabs away from the mic's polar patterns and the group singing. AND turn down the volume so that if will barely be heard on the tracks your recording. But it will still be loud enough for the singers to stay on time.
Either way, Good luck!

I've done this more than once. You want the choir to sing as a choir..that's how you get "that" sound. Just place a pair of mics out front and go ahead and use some speakers. Keep the volume of the speakers as low as you can..and make the mix predominately of the rhythm elements: Drums, Bass, Rhythm Gtr. Also..on the drums..keep the OH's/Cymbals a little lower than usual.

Don't worry about the bleed. If the speakers are behind the mics..the mics (if they're in cardiod) will reject them a bit..and if your using predominately the rhythm elements - all the same ones that will be in the final - the final parts will be so much louder in the mix that it will render the bleed fairly if not completely unnoticeable.

Bleed is not your enemy.
I get some of my best lead vocals with an sm57 in front of the monitors....

P.S. Let the Choir Director where a set of cans to augment the above..that way they here it really well for cues. Also..this means only one set of cans needed.

For some reason, I'm really experienced in this. The bands I do just LOVE adding choirs and gang vocals. Go fig.

Anyway, here's how I do it.

1. Put a guy who knows how to wave his arms in time on a chair in front of everybody. Give him a drumstick to wave around and your best set of headphones. It's okay if this guy is you.

2. Give any extra pairs of headphones to the good singers, or to people you want to feature.

3. Get the choir to rehearse, rehearse, REHEARSE in front of the speakers. This way, they know what they're doing, which makes things a lot easier.

4. Record.

5. To add oomph to the track, do three takes, and vary the positions of people in the room (loud/soft, girls/guys, etc.)

6. For mics, I use Earthworks omnis on giant boom stands. Before I had those, I just used a pair of condensers (large D or small D, it didn't matter), and got them as high as I could, facing down along the middle axis of the choir.

~id

good points idiophone about multiple passes and changing up the position of the members for each pass. I forgot to mention that myself, and it is a typical, and useful tool to augment the apparent size of the choir available.