The Doodlecast app allows you to import pictures and write on them. I've discovered you can save an image from a search and pull it up on the iPad. It's great to use for practice as well as having documented evidence of student work.
I made a quick tutorial on how to get a worksheet jpeg image onto the iPad.
You can also just take a picture of a worksheet for the students to use:
There are many ways to use this wonderful app, here are a few ideas I've used:
Adding on an Open Number Line
Typing Spelling Words and Sentences.
The students can read and record them and then save it.
ScreenChomp and Educreations are two very similar apps to Doodlecast. ScreenChomp isn't as great for images and in Educreations you have to record to save a worksheet. I just have the students record their name and then save it.