To make snow fall you need to create a cloud at the top of the lab using a humidifier to pump in air saturated with water vapour.
To get small droplets to form and for the snowflakes to grow, there need to be nucleation sites in the cloud (small particles around which water can condense and freeze).
The easiest way of doing this is to introduce some smoke into the cloud. Tiny ice crystals grow rapidly in supersaturated air, and fall very slowly. As they grow larger they fall faster, overcoming air resistance, so the higher the lab is, the bigger the snowflakes.
If you can't afford to build your own cloud lab, you could try creating another interesting ice phenomenon - ice spikes. Fill a household ice-cube tray with distilled water and put it in a freezer. Often, vertical ice spikes grow out of the cubes. The water freezes from the edges and the surface of the cube can be almost frozen, with just a small hole. Freezing water from the middle of the cube expands and pushes through the hole creating small spikes.
Where to find it
For more on growing snowflakes visit http:bit.lyAwL4. To get students to develop their own snow and ice experiments, try stap1987's "Investigating Snow" lesson plans. Use fun activities to get pupils thinking about changing water states with Wordy William's "Water Cycle" activity pack.