That's a good ballpark for a high school kid. Also, I'm glad that a libertarian got into teaching instead of the usual commie trash. (His lingo makes it as clear as a Ron Paul tattoo.)

On a deeper level, it isn't a traditional fiat currency: we're operating on what is often termed the 'petro-dollar' system. Long story short, if you want to buy commodities from the world, you want to do it in dollars. This is usually most evident with the oil market and we get particularly frisky if the relationship between our currency and oil gets threatened (hence the name). This is why we can print money with abandon without hyperinflation, btw. All that money we print ends up overseas, being used to buy up those commodities.

So, while it's a good way to explain the system as a primer without confusing basic economic principles, in reality it's very different. Abundance is actually what gives the dollar a lot of value, but only internationally. The Fed orders money from the Treasury and hands it out, but their affect on value is limited to interest rates (btw they have done nothing but lower the value of our money since their inception): the people proclaiming value in this situation are the US government as well as governments worldwide who rely on the dollar to purchase goods. Our currency is backed by no single commodity, making all this fairly untenable in the long term like all fiat currencies. Just remember: historically the "long term" could mean a couple hundred years, so long as bread and circuses stay off the public books.

(Full disclosure: I am probably a bigger gold bug than your teacher was, but I love everyone being fully informed almost as much as I love gold.)