Sherry,
You have a great opportunity to show your students, with hands-on experiments, why some materials conduct heat (or cold) better than other materials.
The "thermal conductivity" coefficient k describes the thermal conduction properties of different materials. A concise description can be found at
http://en.wikipedia.org/wiki/Thermal_conductivity
and, in addition, there are numerous posts on Science Buddies with discussions and experiments on thermal conductivity and insulators.
Do a 'search' on the Science Buddies site and you'll find lots.
In general, the lower the thermal conductivity value for a given material, the better that material behaves as an insulator.
From the wikipedia site, we find:
thermal conductivity for glass: 1.1
" " " stainless steel: 12.1 to 45.0
So these values support the hypothesis that glass will be a better insulator for the water, slowing down the freezing compared to a steel container.
In your experiment, it's very likely that the steel container will have thinner walls compared to the glass - the thicker glass wall will also slow down the cooling effect. So there are two effects that you can point out for the better insulation properties of a glass container, compared to a steel container, that will insulate the contents and slow down the water freezing effect: the much lower thermal conductivity and the thicker glass walls.
Good luck and have fun!
Peter Young