Having a job is not necessary but shows that you can handle juggling between school and work (while maintaining a good GPA) and also work ethics. That said, I have zero actual work experience. XD I only volunteered just like you did and got interviews at all the schools I applied to (only 2). I think it helped that my GPA was above average. So no sweat! There are plenty things the interviewer can talk to you about besides flipping burgers at McD.