I am surprised that the children are being asked to sit still in a Motessori environment. That said, I found, with my older son, that a structured environment with consistent rules are very important for him. He starts having meltdowns in environments where he doesn't know what's coming and can't anticipate change. DS8 has Aspeger's, but all children have more or less of a need for rigidity. Is it possible that your child might be having difficulties because of the looseness of Montessori?