My daughter went to a Montessori until she was in 2nd grade. Her friend went through 5th grade. They are both together again in 6th, in the same middle school, and her friend is really struggling with math.

FWIW, my daughter also struggled with math coming into 2nd grade out of Montessori, but it being second grade math, it wasn't hard for her to catch up.

Also, something to watch out for: Montessori is not a registered trademark, so any school can call itself a Montessori school, without offering a Montessori education.