I agree with Dave above ... standing/sitting in front of a bunch of teenagers trying to get them to understand the basics of a topic is scarey the first few times .. you get to relax a bit from then on.
Once you get used to speaking at events/lectures etc it's easier to hold and associate with a small audience, as you can involve them in some way .. you will probably have to involve your audience visually so - if i were to do this talk - i would take in some visual aids to assist.
Also, Dave's right again ... if you have to talk for 15 mins .. get a 25-30 minute talk sorted and have a watch on the table to look at with your notes ... it doesn't look good if you're always looking at your wrist for the time .. just looks like you want to get out of there and the audience will pick up on it and their interest will waver (ask me how I know that

)
Depending on the level of knowledge of the audience (i'll bet there's some that have no idea what goes into making a model)......here's a few ideas you can either dismiss or take on board if i may offer them.
Take in a new, unbuilt model in a box and use this to explain how the model is first formed by showing the sprues. (maybe show how you take the pieces off the sprue explaining the importance of damage limitation to each piece etc)
Explain in brief why the plans are written in the way they are (i.e for the order of the build)
have a selection of tools with you, and maybe some tools you have inovated - i'll bet there's a few modellers that will get some ideas from you on that one.
Have a selection of paints, glues etc and give a brief summery of their capabilities and applications
Maybe touch upon scratch building and give a brief example of what you can achieve with scraps and odds and sods.
Inject a little humour by mentioning that we use ordinary household chemicals etc to build our models (i.e. Klear) maybe crack a joke about the missus having to "infiltrate the den to look for her cleaning stuff so she can get on with the housework"
you could take another, larger, finished model in to show weathering effects and explain this process.
If possible you could ask the "officials" if there's enough time for a little "question and answer" (if the audience is small enough) at the end of the talk
Mention this when you start your talk and the audience will show more interest in your talk as most people like to participate in these situations - if only to show off a little with their own knowledge, it'll help your relationship with the audience a lot though as they will be paying more attention.
Above all ... have fun ..you'll want to do it again i can assure you
