I went to a Montessori school through the end of grade 7 and then transitioned to a traditional school for grades 8-12. From what I can remember from the transition, it was certainly a bit jarring but ultimately I was able to adapt to the new system fairly quickly and got good grades throughout the rest of my high school education.

Regarding the Montessori experience, I have fond memories of the 7 or so years I spent there. Although it's impossible to tease out causation, I generally attribute my love of learning and personal autonomy to my years there and very much think it was worth it for me.