Many venues do not allow flash photography because it can interrupt the performers, so check with the school or venue first.
This is so true. Many people find a flash going off ..... constantly..... during a performance to be intrusive and annoying and interrupting others privacy. You will need a good sensor with wonderful noise reduction at a ISO like.... 3200?
And as already mentioned a good fast lens. I have done a few shoots with stage lighting, it is situations like this were you have to use your camera on manual. Auto metering will be confused, I have found a lot of the background is dark, your camera will try to take everything back to 18% Grey. Use your spot meter on your camera to get a light reading, set your camera to manual. As much as the lighting may change, with a High ISO and your manual metering setting, you should be fine.
One last thing, privacy in regards to others on stage. Some schools and/or parents do not allow any cameras. If you are allowed be prepared for some..... disgruntled looks.
I am usually asked to events, so it is not a problem, the teachers and principal know I am there. I have heard some horror stories about misinformation in regards to people taking photos of kids at any type of event. Not trying to scare you off, but information and knowledge is power.