A long time ago, primary schools housed students up to the eighth grade and high school started at 9th grade. There were no junior high schools (as they were called when I attended, and included grades 7-9) or middle schools, as they are now called, housing grades 6-8. Hampton is now looking to revert to the earlier model, putting pre-K to 8th grades all under one roof. The reason? Space.
The district initially wanted to build elementary schools on four sites and then build middle schools much later. But the district does not own enough land to build two separate schools on each campus.
In researching the combining of middle schools with elementary schools, Hampton officials found that a number of jurisdictions had converted to the pre-k-8 model. On top of saving space, there is an added benefit: students perform better.
Philadelphia compared test results from eighth-grade students in middle schools and kindergarten-through-eighth grade schools and found the students in the latter schools outperformed their peers. Cleveland reported similar results after studying sixth-grade test scores.
Districts found parents more willing to stay involved with the schools beyond the elementary grades.
Kudos to Hampton City Schools, of which I am a graduate, for doing what just makes sense.
VIVIAN J. PAIGE | All Politics is Local 
After I read the article it sounds as though there would essentially be two separate schools in one building (That might not be exactly how it works out, but it seems to be the concept). If that is indeed the case, then I can see the benefits.
I believe that it is important to have middle schoolers (which for me was 7th and 8th grade only) get used to having individual class periods and teachers for each subject. Since that is how most high schools are run, I imagine it would better prepare students for their high school years. If that can be done while housing K-8 in the same building — probably by having two different bell schedules — then the concept would be worth looking into.
I do wonder, however, how the eighth graders did once they entered high school (It appears as though the study only went up to how they did in the 8th grade) After all, they would have to take at least some time to adjust to leaving the school they attended for 9 years.
Your last point is an interesting one: I wonder if there have been any studies on how they do in the 9th grade coming from the combined school as opposed to coming from middle school. (Yes, the studies cited only went up to the 8th grade, which is where the combined schools would end.)