I'd like to build a darkroom camera,a simple box with upsidedown image on one side,for school demo,any tips appreciated?

'''''''''something simple, maybe that we could do with an finished toilet roll

It's called a camera obscura (http://en.wikipedia.org/wiki/Camera_obscura), and it's a pretty simple gadget.

Taken from http://www.cameraobscuras.com/faq.html:
"How can I make a simple camera obscura?

Such a basic device is often called a pinhole camera. It is made by cutting a ½ inch hole in one end of a light-tight cardboard box and placing a white paper viewing screen on the opposite side of the box. The imaging pinhole is made in a small piece of aluminum foil that is taped in place over the ½ inch hole.

The pinhole is made in the foil with a needle to produce a clean, sharp hole with a diameter of about 1/100th of the distance from the hole to the screen. For example, if that distance is 10 inches, the pinhole should be about 1/10th of an inch in diameter. Larger pinholes make a brighter but fuzzier image, while because of optical effects, a smaller pinhole also yields a less sharp image.

The image on the white screen may be viewed by mounting the white screen over the hole cut in the side of the box. In this case you view the inverted image through the backside of the white screen. Another arrangement uses a small hole cut in the side of the box that is carefully shielded to keep out stray light yet allow the viewer to see the screen. "