Montessori schools do not follow the assembly-line model of education.
Montessori schools are all private because institutionalized educational systems aren't interested in switching their fundamental philosophy of education. There is one exception, the Milwaukee school system. I'm not aware of other places that have tried the same.
It's surprising, but I actually count 91 public non-charter Montessori schools. Often they're magnet schools or 'academies', and it looks like very often just elementary.
Montessori schools are all private because institutionalized educational systems aren't interested in switching their fundamental philosophy of education. There is one exception, the Milwaukee school system. I'm not aware of other places that have tried the same.