We had some trouble getting acceleration from the Montessori school my DD was in.
First it was a little awkward asking for acceleration, because the school is supposed to follow the student. They should have automatically accelerated her without needing to be asked, but that didn't happen to the degree it should have. The teacher had her working with the kindergarten students when she was 4, but she was ready for more. The teacher was initially dismissive of my requests: "She's already working with the kindergarten students. What more do you want!?" The teacher had a very high opinion of her teaching abilities, and took it personally when I asked her to adjust.
We met additional resistance because they have a certain order to follow when teaching. For instance, they don't teach subtraction (not even single digit subtraction) until the kids are doing addition in the teens, or multiple digits. If you expect reinforcement of a topic you covered "out of order" it will be difficult to find, in my experience. My DDs teacher let me know that she was a certified Montessori teacher, and that she intended to follow the Montessori way. Personally, I appreciate many aspects of the Montessori Method, but it has its problems, too. Some teachers may be fully committed to it, or they might be following school policy that is fully committed to it, which is a shame because there are better ways sometimes.
It was a strange contradiction that they offer a child-led learning environment, but (initially) refused to follow my child's lead even when she advocated for herself. It seems they failed to appreciate that a student can gain the required understanding for a certain task without doing the prerequisite work at school.
Ultimately we worked everything out fairly well, though there were areas where they failed to challenge her despite our advocacy. I am thankful for the accommodations we received, and my daughter benefited greatly from her preschool Montessori experience.