Have them read through the documentation, especially the WPILib Cookbook
http://www.wbrobotics.com/attachment...ibCookbook.pdf
Teaching and using some form of version control system like Git or Subversion is highly recommended with multiple programmers.
Once they've gotten sufficiently up to speed with the language and the FRC conventions, split them up into groups of 2 or 3 and have them re-write the code for the previous year's robot with minimal referencing of the previous year's code. This will give them practice writing subsystems and commands, debugging, but also working together as a team.
Also, try making a few SmartDashboard widgets just for variety