On what rationality-related topic should I give a school presentation?

Maybe 'short inferential distances' would be a good place for you to start?

I would call it "why we need classes in rebellion" or something (to get their attention) and then talk about the Milgram obedience experiment and Asch's conformity experiment. Two classic results that demonstrate the power of authority and peer pressure.

Then get your audience talking about how to foster independence without going too far in the other direction. E.g. teachers who nearly always teach the core curriculum, but once in a while throw out a huge whopper to test if you're paying attention. (This works btw. I had a history teacher who tested our credulity by telling us that the pseudo-documentary Punishment Park was real. I googled it and called him a liar next class, and he gave me an A. The vivid memory of being terrified but right was useful for a few years)

Here are three introductions to rational thinking I like:

http://lesswrong.com/lw/8w1/transcript_tyler_cowen_on_stories/

http://www.magazine.utoronto.ca/feature/why-people-are-irrational-kurt-kleiner/

http://lesswrong.com/lw/fc/you_are_a_brain/

And a couple of my own:

http://lesswrong.com/lw/axn/6_tips_for_productive_arguments/

http://lesswrong.com/lw/3iq/being_rational_and_being_productive_similar_core/

Transhumanism could also be an interesting topic:

http://yudkowsky.net/singularity/simplified

Confirmation bias or the problem of short inferential distances are probably the two that would have the most large-scale impact if people really understood them. Either of those would be good choices.

It's hard for me to gauge your audience, so maybe this wouldn't be terribly useful, but a talk outlining logical fallacies (especially lesser-known ones) and why they are fallacies seems like it would have a high impact since I think the layperson commits fallacies quite frequently. Or should I say, I observe people committing fallacies more often than I'd like :p

I recommend taking something like the K&T paper and adding presentation material. I'm not a huge fan of approaching the presentation with the attitude of 'take something big and compress it' in general: I think 'take something deep, explore it, mention there are lots of other areas this deep' is more fruitful.

Maybe something like this? Basically, the idea that science and psychology can impact both our philosophy and our daily lives- that just like learning human anatomy and diseases can make us healthier, a similar approach can be taken with minds.

It follows this advice by painting a picture of what is possible, getting people excited about rationality, rather than teaching them a single tip that may not be useful in isolation.

I would probably focus on evidence. Why you really need evidence to raise any belief, yes any belief and not just those in "scientific" domains, to the point of attention. That absence of evidence really is evidence of absence. The difference between genuine Bayesian evidence and reasoning by the representativeness heuristic. That no amount of clever arguing will help you reach the right answer if you're starting by writing the bottom line, rather than following the winds of evidence.

In my experience, philosophy students learn plenty about formal logic and argumentative fallacies, not so much about good inductive reasoning.

There are several entertaining videos that demonstrate inattentional blindness. The conjunction fallacy is very easy to demonstrate. Eyewitness testimony isn't reliable. Memories we're absolutely certain of could easily be fake. Conclusion: when we're certain we rarely have reason to be certain (not nuanced but nuance is boring).