If these kids are motivated, their grammar and spelling will improve as they read.
An even into adulthood, they will probably distinguish between "home speech" and "school speech" (later "work speech"). Many folks have perfectly clear grammar in the workplace, then colloquial speech among their friends.
I've known plenty of people, beginning in college, who had the neutral accent of a broadcaster most of the time on campus, but who sounded straight out of the hills, or the hood, when they got a call from home. I do it to a fair degree, though my Southern accent isn't that thick at its utmost. Neither accent is faked -- you just learn to adapt.