Molly, if you enter into a volunteer role then you will indeed be expected to turn up and work, they will be relying on you. I work with several voluntary organisations and each one of them depends entirely on the volunteer tipping up and getting on with the job, the volunteers' work is every bit as important as any paid staff. It's not like a drop-in option - you say you are going to work there in your holidays, you go in. It's excellent to have voluntary work on your c.v. and a reference from a place where you have volunteered is just as useful to a future employer as a paid job. You'll get work experience, it'd be a great idea, but you do have to give it your all - otherwise don't bother. There is no "not attending" when you have said you will, unless you have requested in advance not to go in, e.g. to go to the dentist. It's just like having a proper short-term job.