Adding something else to what Yogiba61 has said, I could also say that if you contact with BurlingtonBooks and ask for specific books for mixed-ability classes, they will provide you with some books called "Diversificaci�n Curricular" (2 levels) and 3 other ones called something like "Burlington Beginners" which are quite good on basic levels, dealing with vocabulary and basic grammar. I �ve worked with them with some of my students (especially those with "Adaptaci�n Curricular" and they are quite useful. They also have tests for all of them.

Hope it helps.