You might need more than 5 minutes.
Start high level with definitions of IM, SEO, SERPs, basic ad types. Working with google services like adsense might make the most sense for poeple because everyone in the class knows what google is...
You could also talk about how google knows or is trying to know everything you do online and is tailoring the ads each individual sees on the serp and within the visited web pages too. I saw a youtube video about 6 months ago that was a dig on google on how it is tying it all together and the video is made to make google look evil. I wish I had the link for you.
Pretend that you are talking to a fresh Grade 10 student who is wide eyed and has no idea what you are talking about. It has to be explained in lamens terms. if you know what I mean. relate the terms of IM to other businesses that are more brick and mortar and branding. marketing is marketing.
Talking about the top 10 IMers is ok, but that is like talking about Richard Branson and Virgin to someone who is deaf. They will have no idea who you are talking about.
I like the idea of talking about your own successes, even samples of sites, actual checks you have received ( or the stub, because you probably cashed it ), or even showing some shots of your current online earnings.... then showing how and where they come from.
In the end... i will bet you will have a few new friends wanting to learn from you. You'll get a new IM apprentice
my 2 cents is free.