How making mistakes helps us learn.
And why do we fall, Bruce? So we can learn to pick ourselves up." -Thomas Wayne, Batman Begins 
"You should remember not to use pen when you're doing your homework."
"Because pencil allows you to make changes. If you make a mistake, you can erase and start over."
I have this conversation with my son a couple of times each month while checking his homework. He gets better at remembering, and then a day will come along when a pen is more easily accessible. And then his homework is covered in blue scribbles as he crosses out incorrect answers.
I prefer writing in pen because the ink glides smoother across the paper. It looks prettier, to be honest. But I believe in using pencil in certain circumstances.
Because it's important to remember that it's okay to make mistakes. Mistakes are how we learn and grow. This is true in all aspects of life. Whether you're learning how to count money in first grade, or learning the pain of heartache, or approaching your supervisor when you need to right a wrong. We make mistakes so we know how not to do things, how not to treat people, how not to act.
And, yes, I correct my son's homework before he turns it in to his teachers. But I don't just tell him what's wrong and make him fix it. I walk him through the problem so he can find where he made the mistake. I don't know how teachers feel about that sort of thing, but I think it's good. He's still learning from his mistakes, and learning that it's okay to make mistakes. We learn from them, and then we move on.