As a data point, I derived some parts of the effective altruist idea cluster during my senior year of high school. The parts I understood then were that earning to give was a way to help people efficiently, and that it wasn't something I should look down on. (I'd grown up with the idea that helping with money without getting your hands dirty meant you weren't a good person - I don't know where I'd gotten that from.) This was triggered by seeing a presentation at school by a charity that said they were a good one because they could help more people with less money (I hadn't connected the ideas of charity and efficiency before that), and trying to figure out what to do with my career after high school and what to do with my morality after becoming atheist at around the same time. I didn't really think in terms of finding the most efficient charity, but I remember attempting to do that using one of those charity overhead websites and giving up because I couldn't find useful information.

I don't think effective altruism should be hard for most high school students to understand the basics of. The difficult part is that no one else is talking about charity that way, so it's hard to come up with on your own. Whether they'll accept the idea is a different problem, but I imagine at least some would like it for the reasons I did (wanting to do good, and wanting to follow one of the earning-to-give sort of careers anyway), or like it because it's contrarian. High school is when most American students are thinking about choosing a career, so it's probably good timing, too.

I'm already plotting out a story that's sort-of effective altruist fanfic, but it's set in A Tale of Two Cities, which isn't very popular with fanfic readers. (The story is about a minor nobleman trying to help his village through famines and other problems. The main EA-related themes are making sure your helping is efficient and actually works, and that you should use your privilege for good. I can go into more detail if anyone is interested.)

I'm not sure what a good EA fic would look like, especially if it's the earning to give angle. It seems a little tricky to make a dramatic story about that, because the donor (who I assume would be the protagonist?) is distanced from the people they're trying to help, and money is one of the least interesting forms of power to read about. Making the protagonist's circle of concern or circle of influence smaller, or writing a pair of characters with one earning to give and one on the front lines, would help with that.