1. When the lights go down, shut up and sit still.
Also, make your children shut up and sit still or remove them from the theater, even if that means missing the show yourself. When the house lights go down and the stage lights come up, you're supposed to give your quiet attention to the performers. Extended noisy or restless behavior during a performance is a big "screw you" to the performers and everyone around you. It also screams, "This is about ME!" almost as much your jumping up on stage to join the kids in their joyful, impromptu performance of the Macarena after the curtain call.
2. Clap politely for everybody and refrain from screaming for anybody.
Sitting and talking while other people's kids are on stage and then pounding the ground and screaming when your kid takes the stage is a big "screw you" to those other kids and their families. It also screams, "I have no class!" almost as much as your third grader ripping off her hoodie and writhing suggestively to music about gangs, prostitutes and God knows what else.
Side note here - my experience has been that the ruder people are to other performers and the louder people scream for their own kids (and this includes air horns and other "hey this is all about me" noise makers), the less their kids actually deserve such grand gestures of appreciation.
3. Refrain from criticizing performers both during the show and immediately after the house lights come up.
First, these children, the ones who look or sound like they don't really know what they're doing, are the ones who made up their own acts. Appreciate that. It's genuine. They're not professional performers. They're goofy public school kids who are excited to be on stage at all.
A second grader hacking out Alouette on the piano is genuine and adorable. A fourth grader alternately rocking and then massacring our national anthem is genuine and (mostly) amusing. Sixteen girls in professional costumes and full stage makeup performing a series of moves choreographed by a dance teacher? Well, that's cute, too.
But if they dance with precision, it's not because they're more talented or deserve an audience more than the second and fourth graders. It's because an adult created the dance, taught them how to do it, and made them practice. In fact, I think the second and fourth graders who get out there on their own and perform without a net have more guts than the kids in safe, choreographed groups. And in a talent show that's short on talent, moxy is important.
Second, others can hear you when you criticize the performers. Who knows if you're sitting next to someone's dad or grandmother or friend? Who are you to publicly criticize someone else's kid? Turning to your spouse and saying, "That sucked big time. Lisa was so much better," is a big "screw you" to the friends and families of that performer. It also screams, "I am a self-important jerk!" almost as much as the bedazzled T-shirt you are wearing, the one with your kid's picture and the words "Star of the (name omitted) Variety Show" emblazoned on the front.
Remember that your kid's performance tortures the rest of us just as our kids' performances torture you. I suggest trying to be positive, but if you've really got to complain, wait until you're somewhere private.
All of the above rules boil down to the same thing: be aware and respectful of others. Understand that this show isn't about you and your kid. It's about us and our kids.
I understand that these shows can be torturous, I truly do, but they're fun for the kids and a rite of passage for the parents, so deal with it with a little class, will you?
We're all in this together. Act that way.