I think you'll are probably taking the picture during their full dress rehearsal, and not during the actual day right ? In that case, you'll probably have lots of time to play around with the setting, etc because the rehearsal will probably last the whole day.
This is a picture of my friend performing at SCH last december.
Exposure time - 1/50 s
F-number - 3.5
Exposure program - Aperture priority
ISO speed ratings - 400
Flash - Flash did not fire [off]
Focal length in 35mm film - 380 mm