First, Lexiles is only one of several readability measures included in the CCSS. They started with that one, but as studies were completed they added ATOS, SourceRater, and several others.
Everyone has to remember that Lexiles (and any readability measure) is a prediction of difficulty, and there since it is a prediction there will be a certain amount of error in it. It will sometimes overestimate or underestimate the difficulty of a text. It does this because it predicts difficulty on the basis of only two variables (word difficulty and sentence difficulty).
Obviously there is more to text difficulty than that. Nevertheless, the predictions tend to be reasonably accurate. Why? Mainly because of the consistency of authors. If an author uses simple words and sentences, he/she will probably organize their writing in straightforward ways, and the cohesive structure, tone, and so on will probably not be particularly nuanced or complex.
But that isn’t always the case. Hemingway tended to use an Anglo-Saxon vocabulary, and are there any shorter sentences in the English language?, but try to keep track of who is speaking across pages of dialogue or to grasp what the characters are feeling just on the basis of the words themselves… Good luck, 5th graders.
As I say, Lexiles can mispredict.
The appendices to the Common Core recommend some good ways of looking at text to adjust their placements up or down a bit. Thus, Lexiles (and the other estimates) can get you close, but then you need to use some judgment. No matter what Lexiles predicts, what do you think about using this text with a bunch of kids? (And remember, readability is only one part of the text selection equation—having kids read about sex or violence or racism, etc. in school may be just as problematic if the texts are easy or difficult).
Another reason the predictions aren’t perfect has to do with the reader. The idea of Lexiles and the other formulas is that we are trying to predict readers’ comprehension, and there can be reasons from a reader’s side of the equation why a text may turn out to be easy or hard. Let’s face it, if the author and I share a lot of knowledge in common, I’ll be able to bridge the gaps that he/she leaves for me. However, If the reader has less of a grasp of the content than the author assumed, then the sledding will be a lot tougher. (Yes, it doesn’t matter if there isn’t much shared knowledge if the writer doesn’t presume such information in the writing.)
That means when you are selecting materials you have to think about what the kids might know and whether this text addresses the topic appropriately or not. Again, separate from the complexity of language: does the text over-explain something kids will already know (boring) or under-explain new topics in clear language leaving the kids confused?
Another thing to understand is that a readability score for a text is just an average. The average will be more accurate the longer the text is (more data, greater reliability). However, many teachers and publishers will estimate the difficulty of a text, but then will have the kids read a particular chapter from that text. Different sections of a text may vary quite a bit (so the overall difficulty for a text may be 5th grade, but the chapter you are actually teaching is 3rd grade or 8th grade difficulty).
It might be a good idea to run Lexiles on the actual excerpts and not to trust that the excerpt is a good representation of the overall text.
Readability measures can be very useful predictors of difficulty, but they do not help one to write or rewrite texts for particular audiences. For example, someone might select a text that they want to use, then they find out that according to Lexiles, the text is too easy or hard for the intended purpose. What to do? It is not uncommon that teachers or publishers adjust the passage, perhaps by replacing some words or breaking up a few sentences, etc. That will change the score (making the text appear to be more suitable), but it rarely improves the situation. It should be easy to not do this one yourself, but keeping publishers from playing such games is a bigger challenge.
Like your district, I’m a Lexile fan, but that doesn’t mean that we should misuse or abuse Lexiles. It is just a tool, and one that can solve problems or create problems. Let's not create them.