One of the Thinkfinity Content Partner--Illuminations--has a number of math interactives for high school students. I recommend that you check these out and consider using them with the iPads. The discussion above that mentions how to use Thinkfinity math activities on the iPad should give you the technical information you need to make these interactives work. Please share which interactives you decide to use and how your students respond to the activities.
I did some Internet searching to locate math apps for high school students. I am not familiar with these apps, but you may find some to try with your students. Not all of the apps mentioned are free.