Would Montessori have agreed with psychologist from the behaviorist school of psychology?
The Montessori method is based mostly on student's self-directed learning. According to Maria Montessori, she found the true nature of childhood, and she claimed that the natural tendency of children is to guide themselves towards their independent learning once all the elements that are needed for their learning process are put in place.
The Behavioral School of Psychology is based on the creation of habit to attain a behavior. BF Skinner, for example, would disagree with Maria Montessori because the premise of his behavioral philosophy is that the only way that someone as malleable as a child could only be able to behave the way they are supposed to not through self guidance, but through the guidance of others.
Skinner would say: "How could children guide themselves to good behavior when individuals have an inherent need for a form of discipline that would curb their instinctive behaviors?". Skinner would go for experimentation with schedules, intervals, and elapsed-time activities for the specific purpose of achieving a specific reaction or behavior from a child.
To that, Maria Montessori would argue that since she found the true nature of children, they would know what is best for them based on their level of interest, their capabilities, and their own initiative.
Therefore, the answer to your question is that Maria Montessori may have not agreed with the approaches proposed by the behavioral school of psychology.