K-8 or middle school? Which is better?

Alex Bloom:

As the Scottsdale Unified School District debated closing a school earlier this year, a parent group petitioned the district to let the school grow from providing pre-K through fifth grade into providing pre-K through eighth grade (K-8).
The group included one parent who said she was terrified to send her child to a middle school, which provides sixth, seventh and eighth grades.
K-8 schools have become the norm in the Valley in recent years, although research remains inconclusive on which school structure is better for students.
Regardless, educators agree that success in middle school is vital. A report released earlier this month by ACT Inc., which administers the content-based standardized college entrance exam, found the level of academic achievement students reached by eighth grade has the biggest impact on college and career success.
“By the time they leave eighth grade and go into high school, it’s too late,” said Al Summers, director of professional development for the National Middle School Association.

From the ACT report [341K PDF]:

However, the most recent results for the 2008 ACT-tested high school graduating class are alarming: only one in five ACT-tested 2008 high school graduates are prepared for entry-level college courses in English Composition, College Algebra, social science, and Biology, while one in four are not prepared for college-level coursework in any of the four subject areas (ACT, 2008).
Current international comparisons of academic achievement show students in the United States at a deficit compared to students in many other nations. According to the most recent results of the TIMSS (Trends in International Mathematics and Science Study), U.S. eighth graders rank fifteenth of forty-five countries in average mathematics score and ninth in average science score (Gonzales et al., 2004). The most recent results of the PISA (Programme forInternational Student Assessment) rank U.S. 15-year-olds twenty-eighth of forty countries in average mathematics performance, eighteenth in average reading performance, and twenty-second in average science performance (Organisation for Economic Co-
operation and Development, 2004).
Recent ACT research has investigated the multifaceted nature of college and career readiness. We first analyzed the low level of college and career readiness among U.S. high school graduates in Crisis at the Core (ACT, 2004). The critical role that high-level reading skills play in college and career readiness in all subject areas was the focus of Reading Between the Lines(ACT, 2006a). And when ACT data showed that many high school students were still not ready for college and career after taking a core curriculum, we examined the need for increased rigor in the high school core curriculum as an essential element of college and career readiness in Rigor at Risk (ACT, 2007b). The Forgotten Middleextends this research. This report examines the specific factors that influence college and career readiness and how these factors can have their greatest impact during a student’s educational development. This report suggests that, in the current educational environment, there is a critical defining point for students in the college and career readiness process–one so important that, if students are not on target for college and career readiness by the time they reach this point, the impact may be nearly irreversible.