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I'm not talking about a few scenes when he was a teenager, I mean literally focusing an entire movie on his youth.
You want an entire movie to focus on young Clark Kent?
As I stated previously, Superman: The Movie devoted roughly an hour of its 2.5 hour runtime to that segment of his life. Man of Steel was put together in a somewhat less linear fashion, but probably had about half an hour devoted to his youth. Aside from the commonality between the two - Pa Kent's death - the former's best feature was either Clark racing a train or getting ticked off and punting a football to the next State, while the latter had a more impactful sequence where Clark rescues a bus full of kids (including the school bully) from drowning in a lake. Aside from that kind of thing, what you're going to get in a Young Clark Kent movie will not be terribly exciting, and will not be Superman; it would be more like a standard high school angst movie.
Origin stories are necessary for comic characters like this, but don't need to be featured in every movie (here's looking at you Batman). Anyone familiar with the Superman character already knows he hails from the planet Krypton, and was raised by salt-of-the-earth folks in Kansas. We don't need Fast Times at Smallville High.