I'm not sure, but I think that there is a hefty fine involved as well.
But seriously, when you are in Av mode, turn the little wheel on the top right side right next the the shutter button. That will make the Aperture change when you turn the wheel. Then the camera will pick the correct shutter speed to give you a good exposure (based on the ISO you've picked). When you put it on Tv (which stands for Time Value which means the time the shutter is open, IIRC) then when you turn the dial you set the shutter speed and the camera will pick the correct Aperture to give you a correct exposure. When you are in M mode (and you really, really should go out and practice with your camera in M mode, its the best way to learn), then the dial will change the shutter and to change the aperture you must hold down the "Av +/-" button in the upper right hand corner of the LCD screen while you turn the dial. This is the same button you use to change your Exposure Compensation (if you don't know what that is, there will be another fine imposed - jk).
Both the shutter speed and aperture can be seen on both the smaller LCD panel (the one above the larger one where your pictures show) and through the view finder. Your numbers will be the shutter speed first and then the aperture. Something like "40 5.6". That means your shutter speed is 1/40 of a second and the hole in your lens that lets light in (the aperture) is open to a size of 5.6 (remember that a smaller number means a larger opening and more light is hitting your sensor - so 5.6 lets in more light than 8.0 and 1.8 lets in more light than 5.6).
For a school production with low light, you want to make sure that your shutter speed is fast enough to stop the motion. If they are just standing there singing, you only need something like 1/60, if they are dancing you might need 1/250, if they are really running around, you might need 1/500. In order to get shutter speeds that high you usually have to open your aperture wide open (smallest number) and boost up your ISO. If your aperture is too wide, you could end up with a shallow depth of field. If your ISO is too high, you could get noisy/grainy pictures.
There are plenty of people around here who can clear up all the misinformation I've just laid out, but the jist of it is pretty close.