[Kelly]

Your 30 person limit seems ridiculous. You should certainly be allowed to replace people who drop out. How hard have you tried to talk your school district out of it? Have they given you any reasons why? Perhaps you should propose a "travel team" that is limited to 30 people and allow anyone who wants to help build the robot. That way, the travel team could be determined after the build season and after you have determined everyone's commitment levels.
To actually answer your question, encouraging diversity on your team seems like a reasonable goal. Doing so by admitting students who have already proven to be unreliable is not a good way to do so. These particular students won't make your team more diverse if they don't show up.