The ability to express oneself articulately in writing is related more to the things you mention, like clarity of thought, and also self-confidence, than it is to being taught technique. Oh, well . . .
Some kids are natural good spellers.
They can't help it. IF the motivation is high enough. For instance, the first time I opened Shakespeare with kids I was blown away. They understood it all without a lot of explication. No one needs to comment on another's writing unless asked.