One of the classes that must be completed on the iOS Path for Code School is Core iOS 7. When this course was published, version 7 was the most recent iOS release and the IDE used was XCode 5 from Apple.
This course may be done using XCode 6.x, and most of the exercises will run on the iOS 8/8.1 Simulator that comes with this version of XCode. However, due to some changes between iOS 7 and iOS 8, some of the exercises will successfully compile and run, but the associated tests will fail.
The solution to this problem is that the iOS 7.1 Simulator must be added to XCode 6.x. This is done through XCode Preferences on the Downloads tab. If you are using a MacBook Pro or MacBook Air, make sure that it is plugged in before attempting to download anything through the XCode Preferences interface. If you are running on battery, the download never starts and no error message or warning is given.
Also, before selecting Test in your application, make sure that the active scheme is a device running iOS 7.1. The iPhone 6 and 6 Plus options only run on iOS 8, so choosing “iPhone 5s (7.1)” should work fine.