As a private teacher I've been using New Headway - Oxford Books for over 4 years. It was first introduced to me by my great friend Galaor Bortoletto, who has worked with this material for much longer. Year after year, Oxford has created new tools to help learners improve their abilities and find a way that best fit students' profiles.
The 4th edition is full of fantastic digital tools for both students and teachers. One of them is the online practice that provides a huge number of exercises that are automatically corrected, giving the student instantaneous feedback. These exercises envolve grammar, vocabulary, everyday English, and much more.
Even if you don't use this material, it's a nice recommendation for your students.
Enjoy.