1. The power of compounding. Which will work against you just as quickly as it works for you. Do you want to work to pay your wages to other people to cover stuff you already bought? Or do you want to be the guy who provides the money and then gets paid by other people for the privilege? And "saving" $100 at 1% is stupid if you owe $100 at 22% -- do the math, show them how much worse off they will be (this bleeds into opportunity costs, which is another important point).
1.a. Just because you can cover a payment in your monthly budget does not mean you can "afford" something.
1.b. If you don't have cash to pay for something now, what makes you think you'll have cash next week, or next month, when the CC bill comes due? You have the same job, the same income, and now you're adding a new "thing" to make payments on + interest.
2. To go along with that, basic building blocks of our economy -- a stock is a small piece of ownership of a company, and owning it gives you the right to a small percent of every dollar that company earns for the rest of its life. Just for owning it. A bond is lending someone money, and then they pay you back more just for the privilege of using your money. Which circles back to 1: do you want to be the guy sitting on a wad of cash, who can use it to buy businesses that will then give him more cash, or who can lend money to some guy who wants to buy a new phone right now? Or do you want to be the one paying 20% more for a toy, just because you can't be bothered to save up the money first?
3. Every single company has hired a bunch of smart, highly-educated people to try to convince you to buy stuff you don't need, because it's cool, or everyone has one, or your life will be so much easier, or you work hard and "deserve" it, or whatever. Realize that they don't care about you -- their job is to separate you from as much of your money as they can. They think you're dumb enough to fall for it. It's your job to prove them wrong.
4. Pay yourself first. From day 1. If you've never seen it, you won't miss it. Added bonus that you'll have less money to hand over to advertisers.
5. What taxes really are -- the money our government uses to provide the services and enforce the laws -- and what they are actually spent on. E.g., people always seem to think that some ridiculous amount of money goes to foreign aid and the like. Compare the government revenues to the budgets and the national debt, and ask them to consider whether we should cut spending to balance the budget/decrease the debt, and if so what they are going to take it from (once they see the "real" numbers).
6. How to fill out a 1040EZ.
7. The Shockingly Simple Math. It is their choice to decide how long they want to work for The Man. The less "stuff" they need, the sooner they can be free to do what interests them.
8. Take everything you think and feel with a grain of salt. Talk about the recent science showing how the parts of the brain that control decisionmaking don't fully develop until @25. Kids are brilliant and energetic and primed to fly off into the world and feel immortal, because that's exactly how your brain and hormones are supposed to make you feel at that age. And strong emotions actually shut down the parts of the brain that think logically and can find the best solution. Figure out some guardrails to impose on yourself to avoid doing something that could totally fuck up the rest of your life before you are old enough to know better. Set some basic ground rules for yourself that you do not violate, ever (don't drive after drinking; always use a condom; don't mix mood-enhancing substances; don't bring a weapon to school; limit bad choices to one at a time (e.g., don't have sex while drunk or high), etc.).
9. Everybody screws up sometime. What will determine your success or failure is what you do next.