I'm pretty much in agreement with EricH. Talking about FIRST is a great way for students to develop presentation skills.
In some cases, it may depend on what the goal of the presentation is. In my mind there are a few different "levels" of presentations, which you seem to have hit on a few of them.
1.
Public/Community Demonstrations - This is a great way to let all of the students get involved. Often you want to have your leaders or most outspoken kids do a 2-3minute overview for the whole crowd, but then all of your students can individually talk about their experiences in a more one on one setting with the crowd. Have some talking to people, some explaining how to use the controls, some tossing balls around with the little kids, etc.
2.
Fundraising/General Sponsors - Many teams do Patron Drives, where they collect donations from different businesses in the community. I have linked
1511's page, as they have a whole host of documention you can borrow. This is a great way to raise FREE money, and to get the word out in the community. Most teams will have all of their students participate in this as well, though it helps to run some sort of "training" to educate the kids on what to say. Kids can go together or with their parents or whatever works, but its another great way to get students presenting about FIRST.
3.
Pit Crew kids - not everyone thinks of this position as a presentation position, but 95% of the time it is the Pit crew students that have to sell your robot and other facets of your team to the judges. When you get to build season, I highly recommend watching for which kids are good at talking about the robot and what your team has done and have at least two on your pit crew that can really help win over the judges. This to some extent can be "trained" as well.
4.
High donation sponsors - this can be a presentation intended to gain the sponsorship, or a "thank you" presentation done towards the end of the year. Often you will put some of your best students on this, as you want to give a more professional show. Start with who is interested, and narrow down the field by which students seem to be your best presenters (though it never hurts to put a student with good potential who can learn from the others).
5.
Chairmans Presenters - I list this one last because this can be an incredibly high pressure situation, and one that many teams practice for over a month for. Many teams do actual try-outs or interviews for these positions. This is where you want to select the "best of the best". Though often it is good to mix up the grade levels a bit - ie don't have all 3 presenters as seniors if possible.
So ultimately there are tons of ways to get all of your students involved at many levels. Presentation skills are incredibly important as your students go to interview for college and for jobs, and even in the "real world" so its great to get them started as early as possible. I gave a
presentation skills conference at Championships that might be of use as well.
Good luck & have fun!