I was waiting to see if a Montessori trained elementary or middle school teacher wanted to respond. Anecdotally, I can tell you that I attended Montessori schools as a child and had no issues transitioning to high school (where I was always in gifted and talented programs, CP and IB classes, and made Deans List) and went to a well- ranked college (Emory University) for which I felt adequately prepared. My sisters had similar outcomes.
As a primary Montessori directress, the children I have taught have gone on to very diverse educational environments (private schools, public schools, gifted and talented magnet schools, and continued on to Montessori schools). Without exception, their parents have always reported to me that their child was adequately prepared (in most cases, the parents tell me that the child was academically over-prepared, particularly in language and mathematics, but that they felt it helped them to focus on the social transition).
Most of the talk about Montessori children transitioning is purely anecdotal, but suggests that the transition is rarely problematic (however, there is no way of knowing if the same child would have fared equally well or better in a different environment). One recent study, the Milwaukee 2003 study, attempted to answer these questions more scientifically. In this study, children were admitted to a Montessori school by lottery and attended the school from the age of 3-11; their outcomes were compared to that of children who attended a "regular" public school. Of the children in the Montessori school, over half were admitted to Milwaukee's top four high schools, the children scored significantly higher on state standardized tests, and they scored higher on ACT tests (particularly in the areas of math and science). You can read this study yourself: Dohrman, K.R (2003). Outcomes for Students in a Montessori program
. Rochester, NY:Association of Montessori Internationale/USA
Montessori is one of the only "alternative" education methods to have undergone such extensive scientific testing.
Additionally, this study and several others supporting Montessori education and outcomes are discussed at length in Montessori: The Science Behind the Genius
, particularly Chapters 1 and 10 (Chapter 10 answers your question at length).
I hope that helps!