We use a multi-step text leveling process to assign grade levels to each text we publish on CommonLit.org. We consider quantitative measures, like length and Lexile level, as well as qualitative factors, such as interest level and background knowledge demands. We also use the SCASS informational and literary rubrics to evaluate text complexity.
For example, "That Evening Sun" by William Faulkner has a Lexile score of 570L, but is listed as 12th grade because of the mature content and historical background knowledge needed to understand the meaning of the story. Conversely, "The Fox and the Lion" by Aesop has a Lexile score of 1210L but is listed as 5th grade because of its short length and simple moral content.
When looking at Lexile levels, keep in mind that students should be able to access increasingly higher Lexile levels throughout the school year. Lexile's Lexile-to-grade-level ranges take this into account, by showing different ranges at different times of the year: https://hub.lexile.com/lexile-grade-level-charts.
We believe that teachers know their students best, so use our grade levels as a guide when you select texts for your students. Research shows that students grow as readers when they have frequent opportunities to read texts that are on and above grade level, and that provide the right level of challenge.
If a teacher of 8th-grade students sees that a Lexile level is high for a reading passage, but that it has an 8th-grade level, they should anticipate students possibly needing support with the language of the text.
There are also legitimate reasons to assign a text that is below a student's grade level depending on the purpose of reading the text. For example, it would be appropriate for an 11th grader who is reading on grade level to independently read a "7th-grade" text that provides background knowledge because the low Lexile score allows the text to serve as a quick refresher on a subject.