My daughter's son was in Montessori from 18 mos through 1st grade. He was so focused and independent that he had a very difficult time adjusting...or should I say the teacher. When she would call his name, he was focused on his work and frankly just didn't hear her. Got in trouble for being disrespectful when that was not the case. As well as he didn't have his name on his paper and got counted off 10 points each time making all of his perfect papers down to a B. He was used to his "slot" at Montessori and didn't have to put his name on top.
Further, he get up when he wanted to take his paper to the designated spot. It took a good 1/2 semester to get in the groove and his grades suffered and felt the "talkings to" unfounded.
My daughter (the other one) worked in a Plano Middle school next to a teacher who used to teach second grade. She told my daughter she always dreaded the Montessori kids because they were so independent and worked at their own pace and was either a little behind, way ahead, or just plain bored to the routine of a public school classroom. They "always gave her trouble".
So that's what I know. It's hard on both the child and the teacher. If only we could all afford Montessori all the way to graduation. So for what it's worth...