There are probably around 15 basic economics concept that appear regularly in life. Since high school students should have observed them in their own life, the purpose of the education would be mainly to formalize the concepts and explain how they work on a macro level.
Off the top of my head, these are the concepts I can think of:
1.Scarcity/limited resources/limited choices and related concept that no choice is perfect. That leads to the related concept of Pareto Optimality, which is the closest to 'perfection' that can actually be achieved.
2.Opportunity Cost
3.Utility theory (diminishing marginal utility/resources going to those who are best able to utilize them)
4.Difference between marginal analysis and aggregate analysis and why decisions should be 'made at the margins'
5.Externalities
6.Network externalities, or network effects, which I believe is a similar concept as the 'symbiotic relationship' in nature.
7.Time lags
8.Difference between the short run and the long run. (People have more time to adjust in the long run and, presumably, learn from their mistakes in the long run.)
9.Market failures
10.Asymmetrical information and the related concept of signalling
11.Sunk costs.
12.Normative vs. Positive Statements. A good economics teacher will tell you the only appropriate response to a normative statement is "Why?" (And a bad economics teacher will tell you "Leave Norm alone!")
I would add Bastiat's 'broken window' fallacy - saying that for instance Hurricane Katrina or Libya post Gaddafi was a 'business opportunity'. It is the forerunner to the actual term 'opportunity cost'.
His economic sophism on the candlemaker is a good illustration of crony capitalism.
The Austrian school's subjective theory of value is perhaps one of their most astute observations and expands on the connection of prices to the marginal utility theory of David Ricardo and other classical economists. Obviously also if one is astute enough to economically rip apart the labor theory of value which is essential to Marxism. And the subjective theory of value is a great way to look at that - for instance a diamond is valuable whether you needed to mine it from South Africa or if you found it randomly. The price you pay is when the item you want is valued more than the money you are going to hand over - not equal.
