It's a good practice to talk small and just work along their curiosity, longer talks tend to get people tired, while shorter bring curiosity the most. A few demonstrations could do the trick.

 

Prepare something for everyone. Some of the students might have no clue what you talk about, but kids nowadays are smarter than us at that age.

 

It's great to talk about the teams and the things they can do inside the community. Here is an old wikipage I built with some videos and stuff, it's way out of date (2010) but I guess you can have some videos and stuff to leave playing while people talk to you. [0]

 

[0]  https://fedoraproject.org/wiki/User:Tatica/Laptop_show

Best wishes to all!