Many children spend only their preschool years in a Montessori classroom. Others complete the elementary grades before transferring to another—usually traditional—school. A smaller—but growing—group of students stay with Montessori through secondary school.
A child who transfers out of a Montessori school is likely to notice some differences. For example, instead of choosing his own work to investigate and master, he might have to learn what’s on the teacher’s lesson plan. Instead of moving freely around the classroom, there’s a chance she’ll sit in an assigned seat. Instead of learning in a classroom with a mixed-aged grouping, it’s probable that she’ll be placed just with students her own age.
Fortunately, children are adaptable. Poised, self-reliant, and used to working harmoniously as part of a classroom community, students who transition from Montessori typically adjust quickly to the ways of their new school.
-Excerpt from the American Montessori Society, https://amshq.org/Family-Resources/Montessori-Education-and-Your-Child