This post will highlight two whiteboard apps; one very simplistic, and another with more advanced features.
AWW or A Web Whiteboard is a very simple whiteboard app which can be used on about any device. It allows you to draw simple sketches and drawings. This whiteboard solution works best on touch enabled devices.
Realtime Board offers more advanced features such as pictures, mock ups, drawings, videos, sticky notes, and even Google Drive files. More so than AWW it is designed for the traditional computer or Chromebook interface. Realtime Board has a strong desktop publishing feel throughout its inteface.
Both of these apps could be simple ways to add whiteboard features to a classroom projector setup.