It is not my style--nor really helpful to students--to teach grammar in isolation. If I notice a particular trend in student writing, I will address it in class. If the problem is more wide-spread, I will choose whole-class instruction. If the problem more so relates a few students--or even just one--those students will receive additional instruction in their area of need.
However, sometimes, I like to review a grammar rule by looking at it as an error out in the real world. I have a rather extensive collection of photos, brochures, documents, etc. of misspelled words, improperly punctuated sentences, and so on. On occasion, I will share these with students, and we review the importance of clear written communication.
Lynn Truss's The Girl's Like Spaghetti and Eats, Shoots, and Leaves: Why, Commas Really Do Make a Difference! are two of my favorite resources to see two sentences side by side that have the same words--but different punctuation. And, then, they have totally different meanings!
Here are a few examples of my photos:
And here are my students reviewing (and correcting) some of the examples:
I love this picture of MK thinking about the error:
And here, she's got it all figured out!