http://www.raywenderlich.com/tutorials
Here's one specifically for High School students:
http://www.raywenderlich.com/5600/ios-for-high-school-studen...
And another, creating a text adventure game:
http://www.raywenderlich.com/11079/ios-for-high-school-stude...
And here's Apple's:
http://developer.apple.com/library/ios/#DOCUMENTATION/iPhone...
There are bunches out there, though--would love it if you report back later on what your favorites were!