I had never heard of the schwa until I did TEFL training. It's taught to English learners, but not to native children as part of phonics. I'd guess (?) it's not part of a PGCE either - only if you study linguistics or something like that.
I think your question "How can you solve a problem when you don't know the problem exists?" is the issue. Children don't know it exists, parents don't, teachers may not.
Spelling voice sounds like what I tend to do mentally. I've always been quite good at spelling and tend to "hear" a word as it is spelled in my head rather than the way it would be said out loud. For example, would has an L sound in it, pronounce starts with the word pronoun, etc. I'd not say these words out loud like this but mentally that's how I think of them when I am spelling them.
Schwa sounds like "uh" and it's the sound of -er (in non-rhotic accents) at the end of mother, butter, farmer and so on. It's also the sound of A in woman, O in police, both Es in recipient (though in my accent the first e sounds more like i in pig), Y in pyjama (I assume why the American spelling is pajama). In English we tend to have stress on multi-syllable words so to use recipient as an example, you say reCIPient, it would sound strange if you said RECipient (sounds like recipe - unt). Sometimes the stress changes the meaning of the word e.g. RECord vs recORD. Often vowel sounds in the unstressed part will take on the schwa sound.
But also we do it with the "small" words in between other words, because we stress particular words in a sentence, if you don't, you sound like a robot. So for example "Do you want a cup of tea?" becomes "D'ya wanna cuppa tea?" No matter how well spoken you are this happens. The stress is on WANT, CUP, TEA. (This is also why people think it's "could of, should of" etc - because the have becomes a schwa and the word of commonly becomes a schwa as well, so they sound identical, even if they are pronounced as schwa + v).)
If you tend to naturally enunciate and/or do "spelling voice" you won't hear it when you try to say individual words out loud. You have to try saying the word in a natural sentence.