This lists the rules for comma usage. This addresses the Common Core standard for 6th Grade Language: 6.SL.2. Demonstrate command of the conventions of standard English capitalization, punctuation, and spelling when writing.
This is from a college professor who is combatting punctuation "myths" kids pick up in school. It's a nice article. I like the how the "commas go where you breath" myth is debunked. I think I'll start using it.