Once you understand the basics of blockchain you might want to learn more about cryptocurrencies. Blockchain and Money is an open course from MIT. The course was originally taught in the fall of 2018, but all of the materials and lectures are still available for free. All twenty-three lectures in the course can be viewed in this YouTube playlist. It is a graduate course so I don't expect that high school students would be able to understand all of it, but an interested high school student could still glean some good lessons from it.
H/T to Open Culture.
Disclosure: I have a long-standing in-kind relationship with Common Craft.