Not all Montessori schools are created equal! Two types: AMI (if Maria Montessori didn't prescribe it, we don't do it) and AMS (that was Italy, that was then, this is now).
(Note "Montessori" is not a registered/copyrighted name; be sure to check affiliation with AMI or AMS... or they may be using the name just to attract customers!!!)
Since AMI is rigid, but AMS is flexible, we liked AMS better... but there is LOTS of variation from one school to the next. Montessori standards allow for parents to observe classrooms. Watch carefully. Are children busy and industrious... and are they HAPPY??? Pay very close attention to interactions between children and teachers. You should be able to choose which teacher/classroom is right for your child.
Basic Montessori philosophy is that children are born loving to learn... so don't quelch that natural instinct by "unnatural" teaching.
(Visualize a room full of infants, with a teacher in front saying "OK, class, today we are going to learn how to crawl. Everyone, on your hands and knees, now ..." Get the picture?)
It is possible for a child to get behind in Montessori... although they usually get WAY ahead. (My son learned all of the countries in the world... including many I had never heard of... when he was five.) The key here is: pay attention. Interact with your child... interact with the teachers and director.
Not all children are the same. A GOOD Montessori school will adapt to fit the needs of the child. (Traditional classrooms require the children to adapt.) Observe your children: are they visual, auditory and/or kinesthetic learners? Montessori incorporates all three methods.
But not every environment works for every child. If it is not a good fit, it could be that the Montessori method does not work for the child... or (since Montessori should adapt to fit) more likely it is due to the limitations of the specific teacher(s). Not all teachers are good teachers!
A good Montessori education is fantastic. Children LOVE to learn, they take responsibility for themselves, they learn respect of self and others, self-control, independence, good self-esteem... lots of side benefits of the process.
And they usually are miles ahead of their peers in learning the academics. Again, pay attention to your child and your school.
Most Montessori schools are great for K-6. For Middle and High Schools (rare) there are so many other issues that you REALLY have to be careful in selecting, that it's probably better to "mainstream" them into "traditional" schools around 7th grade.
Most kids have no trouble adapting. Cute story: a Middle School student got poor grades initially, because she stopped her homework when she had learned it... she didn't know she had do the ENTIRE homework assignment. In Montessori, when you learn something, you move on to something new!
Most students thrive in Montessori; it's better to keep them in until 6th grade, if possible, because they will probably get very bored having to do work over and over when they know it already. Better to switch them when your school system starts having the students move from class to class, so they can choose classes/levels that will keep them challenged.
Hope there's a GREAT Montessori school in your area!!!