Reading tests vary from state to state so comparissons are worthless. FWIW, many states are making their tests harder. This naturally leads to lower scores. When scores in Texas start coming out for this year, they will likely be lower because they just made the test harder. I'm sure that people will point to this as a sign on the school system getting worse when nothing has really changed other than the questions they are asking the kids.
One of the major factors that is changing education right now is the number of students with a terrible vocabulary. Most "reading tests" are actually measuring reading comprehension. Having a poor vocabulary makes passing a reading comprehension test impossible for way too many students. We simply have too many students that are never read to or talked to from the time they are born to make up the gap in school. We have soooo many students that can perfectly read a paragraph but not be able to answer questions that ask them to infer. It's next to impossible to infer when you don't know the meaning of 13 words in a paragraph.
Here is research conducted over many decades that all points to the same problems with vocabulary. The only difference now is that we are getting more and more of the lower students in our schools from day one.
First-grade children from higher SES groups know about twice as many words as lower SES children (Graves, Brunetti, & Slater, 1982; Graves & Slater, 1987).
High school seniors near the top of their class know about four times as many words as their lower-performing classmates (Smith, 1941).
High performing third graders had vocabularies about equal to the lowest-performing twelfth graders (Smith, 1941).
This summs things up nicely in regards to reading and vocabulary:
The end result is that enriched environments promote vocabulary development. Good readers read more, which in turn helps them become even better readers with even larger vocabularies. Poor readers read less, which contributes to their becoming poorer readers with more limited vocabularies. In effect, “the rich readers get richer and the poor readers get poorer.”