I volunteer too, and I can see how something which started with a lot of goodwill has grown over the years into an obligation. Do you feel that you know your class quite well now? - if so, surely you can broach the subject to them? I have a voluntary meeting in my house every couple of months - I offered my house, so I make coffee that evening for every and chuck in a few biscuits - but if a snack was involved, or a Christmas do, I'd certainly be asking them to bring something!