Use the 24-105mm as your main lens and don’t be afraid to raise ISO so you can keep both shutter speed and depth of field where they need to be. For a choir, start around 1/100s or faster to reduce blur from normal movement and singing mouths. Use a fairly wide aperture, but stop down enough to keep all three rows in focus; how much depends on how far back you are.

Manual exposure can work well if the stage lighting stays consistent: meter the light, set exposure, and leave it there. Shoot RAW (or RAW+JPEG), and set a custom white balance in advance to make editing easier.

For the full-group shot, shoot from as far back and as high up as practical so faces in all rows are visible and you’re less likely to be blocked by parents. An elevated rear position is often ideal. A tripod is optional, but several photographers prefer skipping it here because people may bump it and you may need to move quickly.

Most importantly, ask the organizers to announce that you’re the designated photographer and request that parents stay seated so you can get clear shots of all the children.