It's easy to forget when you're knee-deep in modifiers and conjugations that grammar isn't just a bunch of rules and forms—it's the system by which we organize language in a meaningful way. Without grammar, language is an incomprehensible jumble of words. With grammar, they actually convey information, ideas and emotions. Understanding the nature and rules of grammar ensures you'll be able to communicate with other people.
Unfortunately, grammar is increasingly ignored in American education. Kids are bored by it, teachers and parents don't really understand it (or its importance), and so it is marginalized or abandoned altogether. Rebecca Elliott's Painless Grammar covers material that in earlier years would have been the content of an elementary English class, but here it's designed for and presented to middle and high school students.
All the basics are here—punctuation, sentence construction, subject-verb agreement, homophones, etc.—in engaging prose with plenty of exercises and guided practice. A final (brief) chapter on email etiquette brings letter writing up-to-date for 21st century students. And while this is pretty foundational stuff, it's likely your older students still haven't grasped the content, making books like this one a more or less essential element of your students' language arts education; Painless Grammar is an excellent text for bringing them up to speed or getting them on the right track.