[Koko Ed]

First things first. Get over the initial shock that not everyone is gonna love first because it's not for everyone and the world will continue to spin. The next thing to realize is that you don't have to keep them. When they signed onto the team it was like an unwritten contract that you would agree to mentor them but they would also agree to do their part. And if they don't fufill their end of the bargain: all bets are off.
Running a FIRST team is like running a business and you need to ask yourself: if these were your employees would you want to retain them for the work they have done. Once you've answered no point them to the nearest door and tell them "Don't let the door hit ya where the Lord hath split ya"
With the remaining students who wanted to work take into account what resources you have and what you can and what you cannot do and simply your gameplan. Make a robot that does one thing well. If that's gathering balls or herding them or whatever do so.
But don't let these kids win and tear down the team. That's what they want. You cannot let them win.