Depends on where you buy them. I agree with you; some of the so-called Kona that Joann's sells I wouldn't buy either. But I suspect Amy uses Kaufman Kona, which is a much better product. There has actually been a lot of conversation about the Kona name lately, and Kaufman (the manufacturer) says they have a trademark on that name. I've seen bolts of solid cottons in Joann's with "Kona" on the bolt end, but nowhere will you find "Kaufman" on that same bolt, and it's not the same good quality.
If you still would like to use solids for your class, purchase the Kaufman Kona at your LQS and make sure it says "Kaufman" on the bolt. Or, you could use Moda Bella solids, which I personally prefer.