I get asked all the time by family and friends to help them determine if the school they are considering is a "good" school. So often it is not, but unless you know why you are looking for a Montessori school you probably won't know if you have found a good one. Research, learn, and observe. That is the only way to know if a school is going to meet your needs.
There are some governing boards out there that attempt to regulate Montessori schools. Association Montessori Internationale (AMI) was founded in 1929 by Dr. Maria Montessori herself to protect her work. The AMI Montessori Teacher Training programs provide instruction developed by Dr. Montessori and adhere strictly to her philosophy. AMI is not the only foundation that aims to govern over Montessori Education; the American Montessori Society (AMS) is another. AMS was founded in the 1960's by Nancy Rambusch, who was trained in an AMI training program and appointed by AMI to be the American representative of AMI. She held that position for many years before she began to modify the philosophy, veered away from AMI and founded the AMS. There are others, but AMI and AMS are the two main players. Others offer online training, correspondence courses and some even sell the work and materials that is usually done by the students in the teacher training programs. AMI and AMS both certify schools as authentic by verifying that they meet their specific guidelines of what a Montessori school should be.
So that explains how there can be such great diversity amongst schools calling themselves Montessori. It also explains the term "Montesomething". Schools that call themselves Montessori, or use Montessori materials, do not necessarily provide a Montessori education to their students. Schools that aren't certified by a governing board are not necessarily bad schools, and on the same note, certified schools are not all good. Misinterpretation of the philosophy can lead to some pretty horrendous mistakes. For example: Many people think that the Montessori concept of free choice means that children do whatever they want to do. However, in the Montessori philosophy we find freedom and responsibility together. Freedom comes from knowing your individual limits and the limits of your environment. Children who are allowed free choice without a clear understanding of their limits do not learn to make good choices or respect boundaries.
The conclusion to this post is that there are a lot of schools out there that call themselves Montessori, but they are not all equal. Parents looking for a good Montessori program should research, find out what it is that they like about the philosophy and then look closely at schools to determine if they provide a true Montessori experience. Prospective teachers school do the same with training programs. BEWARE of "Montesomething" programs. I have heard time and again that Montessori done poorly is worse than no Montessori at all!