Method materials are designed to meet the needs you suggest. They are perfect for the teacher who is not experienced or trained in selecting sequenced lesson material, or who simply desires to leave the task to another. Methods that I'm currently using include Faber Piano Adventures (was FJH publishing, is now Hal Leonard publishing), Alfred Premier Piano Course, Hal Leonard (especially nice for boys), Noona (but moves along a little faster), and I like the new method put out with Helen Marlais - Succeeding at Piano. There are many more - Beanstalks, Music for Little Mozarts, Bastien, John Thompson, Kasschau (That's what I was taught from and I love it but it's fallen out of favor - too bad.) Another method that comes with a detailed teacher's manual and is somewhat more aggressive is the Celebration Series (was called Oddysey, now called Perspectives). The teacher's manual is invaluable.
Some students are ready to move into traditional literature at about Level 3, some benefit by sticking with the method until it's completed. Some can handle supplemental literature from the beginning of study. If you study the literature explaining the use of each method, rehearse/ review/ study and understand what is being accomplished with the materials, and review to be sure there are no "holes", you should end up with a complete course of instruction.
You can also refer to literature review and leveling books such as Jane McGrath's - I think Hinson has one - Bastien has one. They are reference manuals describing the difficulty or nuances of any particular piece. Put it all together, use your best judgement, supplement where needed, trust your instinct - and you should find yourself in a satisfactory positiion for effective teaching.