Rhythm Lab is a comprehensive rhythm app available for iOS (currently $3.99). Not only does it have an extensive amount of leveled exercises, but the app will allow you to create custom rhythm patterns.
Despite the app being able to track individual progress, manual tracking using this lab sheet makes it easier to assign work and view progress without having to frequently open the app.
This download is fifteen pages long covering 59 different sets of exercises.
For more detailed information and a closer preview, see this post:
Favorite iPad Apps for Music Lab Time
If you would like to write a testimonial of how this product has been useful for you and your students, click here.