I'm confused about the classroom situation - if she's in 4th grade this year why isn't she in the 4-5-6 classroom? What's the grade range in her current classroom?

Fwiw, it sounds like you either need to change teachers if you can, and if you can't, change schools. Montessori isn't for everyone, and not all schools implement Montessori in the same way. If other kids are transferring to other schools... I'd take that as a sign that maybe the school isn't all that great of a school.

We had our ds in an optional school with a philosophy similar to Montessori when he was in elementary - and we had a year where he was matched with a teacher and it was totally not working out. We requested a teacher change, and it worked out well - much less of a fight back from the school administration than I would have anticipated. We approached the situation by writing a letter requesting the change - in the letter we commended the teacher and used a few examples of things we admired about her as an educator (which was stretching it at the time, to be sure!)... and then we politely explained how her teaching style was not a good fit for our ds, listed examples from his classroom experience, and gave a suggestion of a classroom that would work better with the reasons we thought it would work better. Soooo... think about framing the teacher request change not as a complaint against the teacher, but a necessity for meeting the needs of your child.

Re changing schools for just one year and changing again - we considered that for our ds going into middle school, and he's a kid who we were worried about too much change re making friends and feeling comfortable in a new place. One thing to consider perhaps more than the change of schools is where will most of the kids at the school be going when they change again? We had a situation we considered where ds would be at one school for only 6th grade, but most of the kids at that school then move on together to middle school - so he'd be changing buildings/teachers/class setup, but he'd still be with essentially the same group of kids.

Best wishes as you figure it all out -

polarbear