Just wanted to chime in with our Montessori experience as well, and as others have said, it depends quite a bit on how the guide and director interpret the Montessori philosophy. Ours was an AMI certified school, supposedly the "best" in terms of certification. Her primary years went well, and she was allowed to skip K and go to Lower El. Then, they changed guides and directors. Her newly-minted Montessori guide was reluctant to let her go to Upper El after three years (the three-year cycle), believing she should stay with her age-mates. Also, she was not allowed to do any projects or outings with the third-year students, nor use the upper level materials (although she asked).
In short, she began to get bored, frustrated, and restless. We realized we needed to make alternate plans for her anyway, since her Montessori only went up to 6th grade, and her testing showed reading and comprehension skills in the 8th - 12th grade level by that time. So, we transferred her to another private school that allowed her to advance to 4th grade (what would have been Upper El.), and she had no problems academically. Now, some of the subtle bullying that went on is another story. She's now at a gifted school, and is doing well.
I guess the point of this long reply is to trust your instincts with your child. We have heard such wonderful stories from other parents whose gifted children thrived in a Montessori environment, and so we didn't take her frustration as seriously as we should have, I think.
-Celia
