If your school doesn't have a digital camera, buy a fixed-lens "point-and-shoot" camera rather than a disposable. The quality is much better, you can keep costs down by sending films to be processed, and they are reusable.
Planning is essential. Get pupils to make a "viewfinder" using a piece of card with a hole in the middle. This will get them thinking about framing and what the final image will contain.
Stop snap-happy pupils by asking them to draw thumbnail sketches through the "viewfinder". It does not need to be a likeness, just shapes and forms.
Discuss and advise.
A few rules
* Don't use the flash, and try not to photograph indoors. In natural daylight you will avoid the "red eye" effect, harsh colours and unexpected shadows.
* Get pupils to think about what the picture is trying to say and what the most important thing in the image is, and to frame that up.
* Experiment with unusual viewpoints. Don't just stand directly in front of an object - move around it, above it, below it.
* Use perspective to make things more or less important in the image.
Experiment with very close-up images. Moving around an object before you shoot allows you to explore it in unusual ways.
* Don't treat the finished photograph as if it is sacred. By cutting, slicing, pasting, colouring and collaging you can get great effects. It's a well-used art technique and gets great results. Using basic materials, photography can be incorporated into almost any lesson.