The point is that "some of the stuff" doesn't matter– the only thing that matters is that you do the most important thing. Once you've done the most important thing, you do the next most important thing. As long as you keep doing that, you're always in the clear.
I think I've seen this point repeated in many other places. Priority applies to only one thing at a time.
Then I won't be giving this to my 9th grader. Everything he's given (by teachers) must be done.
Teaching your 9th grader to prioritize and make his/her own decisions might be the most useful thing you can do.
I think the point is that if you forget to do it, and face no repercussions or regret that remind you of not doing it, that stuff was not worth doing in the first place.
My to do list only ever consists of life admin things that have a hard deadline, like paying taxes, doctor's appointments etc. Anything else is just unnecessary pressure, and as someone else pointed out - if there's a penalty for not doing it you'll remember without a list, or it just wasn't that important.