Someone wanting to learn how to write well should write to learn something else.
Learning how to be a good writer is not an end-goal. Learning how to write is a matter of tacit knowledge, it's about experiencing. Not just practicing but also confronting yourself with new situations.
If you want to learn how to write well, you must write first. There is a necessity to write more, and a great method to never run out of things to talk about is to write with the intent to learn something unrelated to writing.
The reason is quite simple: teaching a topic implies to formulate well-posed questions and answers. It's not just structuring a presentation so that it can be well understood, it's unveiling the essence of the subject to sublime it.
In other words, writing to teach is interiorizing, because great pedagogy is about radiating emotions, which is exactly what art, and thus writing, is all about.
What about formal education then? It's a writing experience like any other. Professors are organized mentors telling you what to read and what is beauty from their own experience. Formal education is not inherently good or bad, it's what you take out of it that matters.