Although already three commenters suggest talking about inferential distances, I am afraid that it is quite hard to make such presentation both interesting and believable. Telling teenagers that you have to carefully explain the idea on a level accessible to your audience, and perhaps even then the audience would not understand if they lack some important knowledge or experience, hm. The audience (the real one at the presentation, not the one spoken about) would interpret it either as banal "it is useless to try to explain things to idiots", or as simple bragging along the lines "I am so smart that you have to study a lot to understand me". When I was 16, if somebody told me that creationists might not accept my argument not because their fanaticism and stupidity but because some "inferential distance", I would think he secretly sides with the creationists. There is little appreciation for subtleties in that age.
If I have to suggest a topic, take something simple, relatively non-controversial and easy to explain. Pick one or more biases or fallacies and present them together with realistic illustrational examples. Base rate fallacy may work fine. Take some quasi-realistic example, such as cancer testing or court trial, which your audience would consider important. Make them guess the answer - that will make it interactive and therefore more interesting. After they get it wrong (they reliably will) show the right answer, which makes a surprising point. You have all ingredients for a good talk.
Telling teenagers that you have to carefully explain the idea on a level accessible to your audience, and perhaps even then the audience would not understand if they lack some important knowledge or experience, hm.
I think this idea is worth telling at the beginning, but of course, in a best accessible way, and shortly. My preferred way is to describe an ancient setting (for less mindkilling, don't even mention evolution, just say "hunters in a jungle") where any knowledge is easily transferred. If someone says "there are gazelles near the...
My school has a weekly event on Thursdays where someone can give a 15-25 minute lecture about a topic of their choice during the lunch break. The standard attendance is about 20-30, aged between 14 and 18, and some teachers drop by if the topic is related to their subject. It's heavily interlinked with the philosophy department, in that topics are typically about religion or ethics, so the audience is generally more philosophically informed than average. A good percentage are theists or deists, and there's a very high chance that the subject will be more thoroughly discussed in the philosophy club the day after.
In a previous lecture a few months ago I tried to explain some standard biases, the Map/Territory concepts, Bayes, and generally attempted to compress the core sequences into 25 minutes, which despite a lot of interest from the head of the philosophy department, didn't go as well as I'd hoped for the rest of the audience. The problem was that I tried to close too many inferential gaps in too many areas in too short a timespan, so for this I thought I should take one rationality idea and go into detail. The problem is I don't know which one to choose for maximum impact. I've decided against cryonics because I don't feel confident that I know enough about it.
So what do you think I should talk about for maximum sanity-waterline-raising impact?