|I had a similar experience when I told an acquaintance that we were sending our ds to a Montessori preschool this year. She said "Oh! Are you *sure* about that? My Mom taught public school 4th grade for years, and every Montessori student she taught was so far behind. They couldn't concentrate at all!"
I left Montessori by 4th grade, was basically "unschooled" for two years with no schooling whatsoever, and entered public school in 6th grade testing at 12th grade reading level, 10th grade math. I think that that a teacher might have thought the same thing about me - because I was freaking bored and didn't understand either the attitudes in class (sullen compliance, no excitement) or the lack of challenge. So I'd read other books in class, pass notes, get in trouble.
Let's put it this way - if
a child is interested, in a Montessori they can learn the parts of speech at age five (nouns, verbs, adjectives, subject, object, participles..etc). This what was covered in sixth grade. The geometry lessons in my public HS were some of the exact same work I'd done in M preschool (geometric solids, metal insets) and elementary. We learned evolution in first year of elementary (big bang theory) and studied the periods (trilobites are fascinating to little kids). Same in public HS...
Now, if they're not interested, no big whoop - and I suppose a child could theoretically enter 4th grade "behind" by not being interested in ANY work going on in a Montessori classroom (math, reading, writing, art, geo, history, etc.), because the teacher won't pressure. But I find this highly unlikely.