It only takes one teacher to bring a whole class of volunteers with her. Teens can also be involved individually as volunteers. Fit it in along the edges. Scanning in a set of photographs is something that can fill a slow afternoon. Typists can retype a valuable document in a few minutes.
Staff can proofread text to stay busy at desks.
Employees on light duty can be trained to scan photographs. Even if a project is assigned a low priority, it will eventually get done. Projects do not need to be massive. In most cases, anything you do will be the largest such project in your community. The more you do, the more support and assistance you will find.