my daughter is in her first year at a montessori school. she is 4. i plan to send continue to send her there all the way through until she goes to high school. her montessori school goes to age 15.
generally speaking the montessori schools work on a 3 year cycle instead of a one year cycle like the traditional schools work. so typically the montessori school has a primary classroom of 3-6 year olds and the child works and learns everything they typically would in preschool and kindergarten in this primary classroom. then they move into the elementary class and work there from age 7-9 and so on...
personally, i went to a small private school, not montessori, just a small lutheran parochial school, from k-8th grade and then went to public high school. it was a good experience. my mom gave me the choice of going to the lutheran high school or the public high school. i transitioned well to the public high school. my sister chose the small lutheran high school and she did well there.