You could definitely volunteer if they let you do it on a more flexible basis or at least not too many hours in one week. See about even just doing one shift per month to start out if possible. Or volunteer a half shift to cover lunches/break maybe once a week or every two weeks. Talk to the person who oversees volunteers and see what opportunities are available. Be honest about your student status and that you are unsure how much time you will have and do not want your studies to fall behind but that this is something you want to do and maybe they will be willing to work about what is flexible for you. You never know until you try.