Showing crystallisation in the following way is one of the best school experiments.
Illustrating growth of crystals under the microscope: make up several "boiling tubes" of warm to hot saturated crystal solutions, using about 150mm3 of water. Leave the tubes in a heated water bath. (Try copper sulphate, potassium dichromate, any alums like chromium potassium sulphate or ammonium iron sulphate etc.) Meanwhile set up a microscope and focus on some salt (sodium chloride) crystals. Next, replace the slide with a clean one (ideally but not essentially warmed in water).
Now the clever part! While one student looks down the microscope, watching eagerly, the other carefully drips two or three drops of hot saturated solution on the slide with a glass rod directly under the lens. Nothing is seen at first but as the solution cools, the beautiful shaped crystals slowly grow.
Pure beauty, especially with different colours and shapes of crystal! By varying the temperature you can grow small or larger crystals (larger if cooling is slower). This is guaranteed to keep them interested! The Science Year (free) microscope sent to schools linked to the PC might be used, but true beauty is in the eye of the microscope.