Voluntary, by the definition, means to do something willingly and not because you are forced. That is only one of many joys of volunteering; you can do what you admire doing and learn many new things.
Firstly, I think you learn how to work in a team and communicate with its members. It is a useful skill we do not talk about so often in school. Since not every team member pleases you with his work, you get more adaptable, which can help at your workplace later. Secondly, I believe I get more helpful. If you do something, such as help cleaning the park, and not get paid for it, you will more likely learn to help others without expecting a reward.
Many employers only employ people with experience. In my opinion, those gained when volunteering are as or even more important than those gained when working for a wage. In addition, you can get the former ones as a student or when you finish school and not only when you are employed.
My classmates and I recently took part in Dramski festival, a project which allows every class to perform on stage, organised by Gimnazija Jesenice. We learned to listen to each other and cooperate. It also increased my self-confidence as I had an important job controlling the lights.
To sum up, I think doing voluntary work only has positive side effects and that every student should do some. It helps one get experience, learn about teamwork and enjoy what is done. People who volunteer are not paid because they are worthless but because they are priceless.