Hi Prasad,
First off, I'd like to commend you for bringing art programs to areas that may not have that opportunity so readily available. I teach science and dance in a mostly middle class area and it has been difficult keeping arts and music alive in our district.
While I'm a novice (aspiring) calligrapher, my middle school students are around your target age group and I think it's important to appeal to the different types of learners. I know that some of my students are logical thinkers and others are visual learners, for example. The repetitive exercises will be great for logical thinkers, but may bore the more right brained participants.
Calligraphy itself is a project for the individual, but I think it would be great to make a part of your class a group activity. Regardless of their right vs left brained nature, students are always up for working in groups, and I thoroughly believe that sharing their work with each other will entice your more lackluster participants to put more effort into what they are doing
So here's an idea! I think you should divide your students into groups and have them work on their practice guides, but once they are done, have them decorate their exercises by making each line into a drawing of some sort. For example, O's can create a caterpillar, or downstrokes and upstrokes can create the legs of sheep or horses. If they're very creative, each group can compile their work into a book to take home.
It will be a laugh for many students to see what their friends have come up with . I'm not sure if this is too lenient of a class structure for you, but it would definitely be something to try!
Best of luck and know that your students are lucky to have you!