I started saving for retirement as a teenager, as soon as I had my first taxable income job (ie, not a paper route) at 15. I opened an RRSP (the Canadian Registered Retirement Savings Plan, similar to a 401K I think). I put $400 into a 'high interest' RRSP account (I think I was likely earning about 1% apr, so pretty low) but I think it got me started early. Other than the years I was in uni, and had no taxable income, I have contributed into my RRSPs, which are now much better diversified.
You can have other short/medium term savings vehicles that won't fluctuate in value as much as stocks/ETFs, but will earn lower returns, (will be more stable), if you want the money for school in a few years. Unfortunately, as a Canadian, I can't be much more helpful than that. It pretty much boils down to sacrificing the potential of higher returns, for the security of not having to worry that you'll wind up needing the money, when the market is down. But something tells me that you might already know that :)
We have these wonderful Registered Education Savings accounts here, where the government will kick in 20% of any contributions your parents make (to a max of $400/year I think). We don't have kids yet, but we will be contributing the max we can to those programs, because a 20% grant is a pretty stellar return, even if the accounts can't hold that much. And with the way it is set up here, we can transfer any unused funds to another kiddo, or into our own RRSPs, if they decide not to use it (minus any grant money, of course, or everyone would be getting RESP's to roll into the RRPS's).
I'm not sure exactly what age you are, but I've heard that "a random walk down wallstreet" is a very good beginner's investment book. And I learned most of my compound interest stuff in grade 9 math class. Do you have a trusted teacher that might have some more applicable American examples or suggestions?
Anyway, welcome, and good luck! The fact that you are thinking about this before even finishing high school, is a wonderful thing, in my opinion :)