I have had the chance to do what you are doing, and I was lucky enough to have a mannequin loaned to me. I also had some video of people diving, and as the video played I geared up the mannequin, explaining each piece of equipment and what it was for. I included a safety sausage, mirror and whistle, and omitted any knife or spear gun. ( Sorry guys, you can't take those to schools). The kids' comments gave me more ideas for the next presentation- they commented that it was like an astronaut dressing up for space, so I added a picture of an astronaut on a space walk for comparison. After many asked to try on gear that was just too big for them, I started taking kid sized gear from my local dive shop: kid sized bcd and small tank (20 cu,ft. I think), kid fins and mask, and regulator. I ask for a volunteer to try on the gear, and everyone it seems wants to try it. I had time to put it on several. I think the key to these presentations is to be visual- videos are great, and the more "stuff" you bring, the better. I brought the Sea Life camera I took the video with to show that, and kids were very interested in that too. Don't lecture. Do "show and tell," and expect to be interrupted constantly with questions and comments. If a question is asked, answer it right away, don't say "I'll get to that." Presenting scuba to kids is almost as much fun as diving. If you don't have the use of a mannequin, do try to get kid sized stuff for your presentation. If you have to go with one or the other, do kid sized stuff, and let one of the students (or more) be your "model." And of course, underwater video for sure.
In my limited experience, here are the questions I have had, and which I think you are likely to get too:
How deep have you gone?
Is it scary?
Aren't you afraid of sharks?
Can you drown while you are diving?
Does it cost a lot?
How old do you have to be to go diving?
Is it dark in the bottom of the ocean?
How deep is the ocean?
Have a great time, and report back to us on this thread how it went and what worked and what didn't.
DivemasterDennis