I’ve done a fair amount of recordings of orchestra, string ensembles, wind ensembles, Jazz big bands, etc… This being your first time, the key is to keep it simple. What LDC’s do you have?
Keep it simple - 2 mics as a stereo pair is fine. Generally, SDC’s are a better choice than LDC’s. However, you can still get a very nice sound from a pair of LDC’s. Its probably best to stay away from complex stuff like Decca trees, jecklin discs, lots of spots, etc… Not that these are bad, but they add a degree of complexity that you should avoid on your first outing. Also, in a live setting, you need to minimize things that block people’s sight - which is another reason to avoid trees and discs.
Distance from stage - closer is generally better. The closer you are, the more direct, versus reflected, sound you’ll get. You can’t really take reverb out of a recording, but you can add it quite easily. Placing the mic array 10 feet back from the stage would be a good place to start for a school/youth group. However, you should consider SRA (see below) to refine placement.
Height of mic array - You’ll want some height. Ideally a few feet above the conductors head. This is done for several reasons. First, getting some height helps minimize audience noise. Second, it allows the stereo array to be pointed into the center/back of the group. This helps balance the sound.
Stereo Width (Stereo Recording Angle) - This can be a deep topic, and I don’t want to derail this thread. But you should consider the Stereo Recording Angle (SRA). Here’s the very highly condensed version. Stereo imaging is created by intensity differences (volume differences of left versus right) and/or time differences. Angling directional microphones (such as when using an XY array) provides a stereo image by intensity (volume) differences. Spacing microphones apart (such as when using an AB array) provides a stereo image by timing differences. Spacing and angling microphones apart (such as when using an ORTF array) provides a stereo image by both intensity and timing differences. SRA is an imaginary angle coming from the center of the stereo mic array. Sounds within this angle spread out nicely across the stereo image between a pair of speakers. Sounds coming from outside this angle are pinned hard left or right, meaning that they come from one speaker. You want to keep your orchestra in the SRA, ideally the edges of the orchestra would hit your the imaginary lines of your SRA. Some smart people have figured out the SRAs for various stereo mic arrays. Michael Williams was the pioneer here. His paper, The Stereophonic Zoom, explains this well. You can google for it, go to AES website, or try this Rycote link: https://rycote.zendesk.com/hc/en-us/articles/203908232-The-Stereophonic-Zoom-Archived
You want to avoid making a recording where everything is bunched up in the middle of the stereo field. This happens when picking an array with a wide SRA (like 180 degrees) and then placing the array back a distance from the stage. You want to avoid making a recording where everything is bunched up on the sides. This happens when the SRA is narrow (say 60 degrees) and it’s placed too close to a very wide group. If you want to visualize the SRA for different stereo mic arrays, Sengpiel has a great sight for this. Here’s the link:
Which Stereo Array?
XY - You’ve tentatively picked this one. It will work, but you can probably do better without much effort. Also, XY, where the mics are at a 90 degree angle, produces an SRA of 196 degrees. Very wide. Unless you plan on jamming these mics right up close to the performers, you will have the sound concentrated in the middle of the stereo image and nothing on the sides. Also, XY can sound a bit lifeless and dull. XY is coincident and this will not sound spacious as other arrays can. On the plus side, stereo imaging is good because of the ability of the mics to pick up intensity differences.
AB with omnis - this would be my first choice in a room with great acoustics. It will sound spacious and glorious. However, the downside is this will pick up bad acoustics and highlight them. Although I love this technique, I would not recommend this to you for your first outing. SRA varies depending on how far you space the mics apart.
ORTF - this is accomplished by spacing the mics 17 cm apart with a 110 degree angle between them. Combination of intensity and timing differences for creating the stereo image. Imaging is good, and it has a bit a spaciousness to it. This array, while not always the best sounding one, will almost never sound bad. Bad acoustics? Doesn’t matter - it will still sound good. I recommend you try this one as it is almost impossible to screw it up. It’s a very forgiving array. With an SRA of 96 degrees, it would work well in the situation you noted. Given that you have 3 groups going on and each are probably different widths, consider putting this up within the imaginary angle of the biggest group, then move the mic array closer so you’re clipping the sides a bit. Putting this mic array up would be simple. Imagine that the SRA is actually 90 degrees (easier to imagine than 96 and is close enough) and emanates from the center of the mic array. Move the mics closer/further to stage to tweak your distance as you visualize how wide the group will be. As I noted above, starting out at 10 feet from the stage is probably a good place to start.
Other arrays - there are plenty. Not worth considering for your first time out. Keep it simple.
Safety - If this is a live concert, you’ll want to take a few steps to keep things safe. Bring a roll of gaff tape and tape down your cords (mic cords and any power cords). Use a sturdy mic stand to get the mics up. If your stand isn’t sturdy/heavy, then consider using sandbags to weigh the base of the stand down. People bump into stands at concerts. You can use milk jugs filled with water and a bungee cord as an alternative to sand bags. I use a quickloc A-50 which weighs a ton and has solid steel legs. People have walked into it on several occasions and it has not been knocked down.
Bit Depth - 16 isn’t enough. 24 is plenty.
Sample Rate - 44.1 is fine. You can go higher, but its probably not worth it.
Setting levels - You’ll need some headroom, but you don’t want to leave too much headroom if you can avoid it. Aim for peaks to be between -20 and -12. Leaning towards -20 would be a little safer. If you get a sound check or any time in advance, ask somebody to wack the bass drum for the wind ensemble. That will likely create the loudest peak. Adjust from that. Your levels will need to change for each group. The wind ensemble might be the loudest. String groups tend to be quieter and will likely require more gain.
Backups - Live events generally don’t provide for second takes. If you can, have two machines recording. Use a passive transformer based splitter to make the split. Don’t use Y cords or other active contraptions (these are fine, but not for your first time out). Passive transformer splits don’t need power, automatically isolate the recorders from each other, and are dead simple to use. Even a cheapie splitter like the Art S8 is worth having. Your other choice is to throw up a second pair of mics going into a second recorder. Even putting up a simple Zoom recorder as the backup is worth it.
Hopefully this helps some.