There are a lot of great ideas in this thread; I would try to take advantage of all of them (if you had more time). In any case, I am on my districts school board, so I thought I could try to give a little insight.

Greg Marra and Tom Bottiglieri summed it up most succinctly;
Impact and Educational value.

In a more roundabout way of saying that, here are my suggestions.
The school board likes seeing awards as much as the next group of folks, but what they really want to hear is that something they have at their school is making a positive impact on students and has not only educational value, but is also enjoyable.
Having said that, visuals are always nice, but try not to get too "busy". A robot and posters and a PowerPoint, etc... can be a lot to take in for 15 minutes. Having the robot there is, for me, a definite must though. It is one of the most visible, unique products of your team. Other teams have poster boards. Other teams do not have a robot.
Give them only enough background about the game as needed. Rather, tending back to the impact and educational value theme, spend your time with how the experience has caused growth. How has your school district sponsoring a FIRST team helped you? Enthusiasm counts.

Lastly, invite them to see an off-season competition with your team, a Regional event, or the Championships. There is more to FIRST than anyone can express in words. My coach once told me that "The hard part is getting them in the door. After that, it [the attraction of FIRST] works by itself." I am a firm believer in that. Get them involved as fans of your team; the experience is something I wouldn't trade for the world--it will have an impact on them as well. Remember to thank them for their time, and that if the point of your presentation is to garner monetary support, remember to impress that as a last point. No use in having the message get washed out by the rest of the presentation.

I lied about the lastly part. Lastly, lastly (really now).
Mr. Kressly is an amazing FIRST Senior Mentor, and his support has been invaluable to my team. Take advantage of his advice.
Hope this helps!

-Mike