As others have said, check out the credentials of the Montessori school and the teachers. The regulating agencies are not consistent. There are many "Montessori" schools which are only Montessori on the surface. We did not get into our public Montessori school (lottery) but it was a great fit for many gifted kids - but they were NEVER held back by the curriculum that I am aware of. Also, because the classes are 1-3 and 4-6 grades, there is not pres-school stuff in the building. My understanding of Maria Montessori's philosophy was that children should not be limited by the confines of the grade or the teachers' expectations.
I've heard parents who've had kids in Montessori pre-schools say things like, "that's great for pre-school but wouldn't work in elementary." These are kids who have been in schools that I believe are not truly Montessori in philosophy. There is one Montessori pre-school in town that is excellent and a friend with a daughter who just turned 4 showed me the writing that she had done at school. It was cool to look at because it looked sort of what an 8 year old might write, but then you could hear the four year old "voice" and the spelling was more 4 than 8. This program will let her go as far as she wants to before Kindergarten.
All that said, I think the philosophy and approach of the individual teacher is the key.
I am reading a book called "The Element" and there is a line in there: Great schools are filled with great teachers, but there are a lot of mediocre schools filled with curriculum and assessment. (paraphrase)
