Teens sometimes need a little more training and guidance than adult volunteers. The good news is, that if you invest extra time at the beginning, it really does pay off. Teens will rise to your expectations. If you tell them you expect them to act mature, and treat them like an equal partner in completing the library's mission, they will rise to that expectation.
A good process to help you as you are training new volunteers is the following steps, based the Aspen method:
1. I do; you watch.
2. I do; you help.
3. You do; I help.
4. You do; I watch.
Many times, the frustration with volunteers doing their tasks incorrectly is that they didn't actually understand in the first place. We explain something, and ask if they have any questions. When asked if they have any questions, teens will almost always say, "no," either too shy or unable to formulate their thoughts into a coherent question. We then move on, assuming they know what they are doing.
How do you keep this from happening? Taking time to observe new volunteers at work will help keep errors down.