I guess for me this thread ends up feeding in to the idea that gifted students only come from middle class families, when everything I have read suggests this is not the case (though obviously many gifted students do).
If you've got a working class family history, live in a working class area and attend working class schools you're not necessarily going to be exposed to the same ideas and resources that a middle class gifted child would be. Education may not be a priority, leaving school early might be necessary. You might get little exposure to good books. This is normal to you (as others have mentioned) and you don't know that you're wrong. Or you might know that you're wrong but you're not going to adopted the correct usage because it'll mark you as different to your peers.
There is a great book, Gift of the Gob by Australian linguist Kate Burridge that shows that there is a lot of language that we assume is incorrect when it is actually historically accurate. Between you and I/me is one of them (between you and I being historically non-standard, but now the accepted form)
In my own case, I can never get interested in grammar and I am a terrible speller. I know enough to get by, but only just enough to know the difference between and adjective and verb and how to use an apostrophe!
