It's your right and even your duty as a parent to guide the education of your children. Exercise that right.
I am going to echo @Prinx's good answer in mine. In most elementary schools, they just don't care (as you've witnessed first hand.) They probably feel that they have bigger fish to fry, and will let the kids learn punctuation in high school. By then, habits are a bit harder to change, though they changed for me, and I'm sure if your kids care about their grades, they will learn it in high school. However, it doesn't hurt them to know the basics sooner.
Take it up with the principal and the school superintendent. If the high school that the kids go on to use the Chicago manual, the grammar school should as well. But, having done this, if you get no results, teach your kids at home. You're a writer. That's part of a craft you feel strongly about.
There is no end to teaching materials that drive home the importance of commas ("Let's eat Grampa." vs. "Let's eat, Grampa.") and the Oxford comma in funny ways.* Humor is an great teaching tool. On a weekly basis, give them a few hilarious examples of the danger of leaving out a comma and the Oxford comma.
When they have a paper due, proofread it. Yes, it's extra work for you that the school ideally should be doing. But if you can't get the school to do it, do it yourself. That's what having a hand in your kids' education sometimes comes down to.
Just don't badmouth anyone, and teach your kids to respect authority enough that they don't correct the teachers. That way, no one suffers.
*My favorite recent "comma story" comes from the New York Times. In a short review on the occasion of the release of her new book in paperback, This Is The Story of a Happy Marriage, by Ann Patchett, the New York Times reviewer wrote:
These sparkling personal essays, by the author of Bel Canto and State of Wonder, cover the quotidian and the profound: from Patchett’s passion for opera and her stabilizing second marriage to her beloved dog, and her resolve to open an independent bookstore in Nashville.*[bolding mine]
Patchett was obliged to answer in a letter to the editor:
...When highlighting a few of the essays in the collection, the review mentions topics ranging from “her stabilizing second marriage to her beloved dog” without benefit of comma, thus giving the impression that Sparky and I are hitched. While my love for my dog is deep, he married a dog named Maggie at Parnassus Books last summer...
kids of different ages will find some things funnier than others. Choose appropriately.