One of the free apps I found in the top charts under the Education category is called Absolute Board. It's meant to be used to "sketch and take notes about anything in your mind."
I found this app easy to use. There's an exclamation mark icon and if you forget what the other icons do, choosing this one shows all the labels for the other icons. Then it fades out and you can continue drawing, writing, etc.
I liked being able to choose the colors and line size. The eraser function is quick and easy, and you can zoom in and out in addition to changing the line size. You can also save your board screens and send a snapshot via email. I sent some snapshots to myself and found it easy to add a .jpg extension so I could open it in preview. From there you could save it as a .pdf, insert it into a document (or blog post...) and so on.
It seems like a very flexible app that has many potential uses but I haven't figured out what they might be in a classroom setting. Any ideas?? Please post and share if you have thoughts about applications for this app.