Pairing CoDrones can be a confusing process, so I hope these steps help you successfully pair every time:
Install all the necessary software and hardware files for your operating system (tutorials are here)
Open Arduino and upload the default flight controller code which can be found by navigating to File > Examples > CoDrone > Controller > FlightController.
Plug in a battery into one drone and wait until they pair. You do not need to fly it.
Go back into the code and change CoDrone.pair(Nearest); to CoDrone.pair();
Reupload this new code and pair to the same drone again. From now on, that controller is paired with that drone as long as students always use CoDrone.pair(); without the Nearest parameter. It's a good idea to label the pair with a colored sticker so that you know which ones are paired.
Repeat for every drone.