I'm assuming you mean absolute beginners to low intermediate? That's pretty tough to keep all of them involved in the lessons. I think we've all tried to do that one time or another, and usually with limited success.

The common idea is to get the lower level folks caught up with the higher level folks, while still encouraging advancement and success for them also. It also depends on the age group you are teaching. Children or adult, or somebody in between? Either way, it may be impossible to keep everybody engaged in the lessons, especially since you'll be teaching some of the most basic and simplest of grammatical forms to the newest learners. I guess I would try to incorporate a little of both nto every lesson, by starting with some simpler structures, which would be a good review for the stronger learners, and then move to something more challenging for them. The students would need to know right off at the start of the course that they are at varying levels of competencies, and will at times feel frustration over the pace, but that you will include something for everybody in every class. And the regular reminder of that during the course would also be necessary.

As far as books go, maybe you can find a book that has two or three levels in one book, and give assignments from early in the book to the beginners, and assignments from later in the book to the higher level students. That does make it a little more difficult for you to create lesson plans and homework assignments, but that's what has been set up for you: this year's challenge!

And remember, even though some people test at a particular level, they don't necessarily function well at that level. They may need the review or training at that lower level in the class, or they may have had more instruction or remember more as the class progresses.

Good luck!