That does not sound like a good Montessori school at all! Problem is, anyone can slap the name Montessori on a school.

It's important to make sure a school is accredited with one of the major Montessori associations like AMS or AMI.
There is some freedom in Montessori, but it is freedom within limits. My kids have work plans for the week with the works they should complete. They can choose to do all math one day, but at some point they have to do the reading, writing, cultural studies, etc. on the work plan. The teachers are also very good at coaxing a child to work on something they are avoiding. So, say a child is resisting learning addition, but they love to play with dinosaurs. The teacher could use dinosaur counters and make up word problems with the dinosaurs. Doesn't like writing? Give them words to copy that they like--princess, castle, rainbow, etc for girls.
It is actually more common for kids coming from Montessori to be ahead of their peers. They move at their own pace, so some kids can easily be 2-3 grades ahead in a subject. Because the younger kids are observing the older kids working, they will often be very interested in learning more about the lessons leading up to that lesson. I have even had four year olds learn all their letter sounds by watching others doing the work. Suddenly they are just reading words in books.
Good luck with that child. I wonder if there is a learning disability?
Marsha