This Instructable contains a Common Core-aligned lesson plan (in PDF form) and an accompanying PowerPoint presentation (zipped). The slides on modeling and using the shadow tools can be hidden if the students are already aware of how to use the app, or if you would like to walk them through it yourself.
This lesson plan goes along with this Instructable, which walks you through the process of actually modeling and analyzing the garden.
Also, I recently ran part of this lesson plan with a group of middle school students, and they were very quickly able to pick up the iPads and get to work efficiently, so high school students shouldn't have a problem with it.
Oh, and of course, any feedback is appreciated! Please let me know what you think, and post if you have any changes that you think would make it better.