You are approaching a very broad problem with a very personal set of premises. I don't believe you'll find any answer sufficient. None would have for me at (what I'm guessing is) your age. I dropped out of high school weeks before what would have been my graduation, and it took a long time to untangle just what had gone wrong with it and what had gone wrong with me, and which of those things I actually had any influence on.

On the one hand, you'll want to hear that you're right, and that school is pointless. Well, that's kind of true. Public education is a grand human enterprise, and like all grand human enterprises it is a mixture of hubris and groupthink seasoned liberally with ineptitude and half-baked in a fiery furnace of failure. (Served on politics and garnished with voodoo.)

There are certainly motives involved, and results obtained, but you'll never find a precise mapping of one to the other that doesn't break down at some scale. It continues to go on not because it has a singular point to it but because it is a means for many different people to achieve many different ends. It's an ecosystem, and as such it spans every biome from desolate failure to flourishing success. Taking your own experience as highly representative is apt to lead you astray.

That is not to say that your experience somehow doesn't count, only that in order to look at the bigger picture you have to see yourself as a smaller part of it.

(A particularly pernicious variation of this comes with the way you judge the success of your peers. How you feel about their lives might be important to you, but that's a very different thing from how they feel about their lives.)

On the other hand, you'll want to hear that you're wrong, and that you didn't waste several years of your life in school. Well, that's kind of true, too. You may have spent those years in school, but whether it was a waste is entirely the perception you choose to ascribe retroactively to that time. You can turn that off. The fact is that nothing changes either way. Wasted or not, it's gone.

You probably spent more time finger-painting as a child than you can reasonably justify today. Do you consider that time a waste, or just history? Why not cultivate a similar attitude towards your schooling?

I think this gets easier with age, or more accurately, with distance (in time). You start getting insight into how unreliable your perception of what is or isn't important is at the time. This is a heuristic for judging the value of time that I think can only be trained by experiencing the opportunity cost (i.e. by "wasting time" and recognizing it.) The thing you gain from this is never compensation for lost time, but the ability to waste less time making better mistakes on more interesting problems.

There is a lot to be said about other questions you raise (e.g. Q:Why do many people in shit jobs miss high school? A:Because it was structure without responsibility.) but I'm going on at length about this for the (admittedly presumptuous) reason that I think it's very easy to confuse thinking about the hard problem of public education with wishful thinking about the impossible problem of what you might have done with a few extra years of free time. That's the same kind of thinking that leads people to waste time trying to "hack their sleep" so that they'll have "more time". But you don't get more time. Ever.