The Kyoto Scale
The "Kyoto Scale" is loosely based on reported headword counts, but further adjusted by close inspection of the text series, with reference to the "Yomiyasusa Levels", created by Akio Furukawa. The Kyoto Scale divides all books into ten levels of increasing difficulty.
This table below shows a very rough correspondence between the Kyoto levels and other language level criteria. You can use this to determine initial (tentative) reading levels for your students. Note that it is desirable for their reading level to be somewhat below their actual current ability.
Chart equating the reading levels of some major reader series