Wow - had a similar issue with our Montessori pre-school. DS7 was 3 & doing easy reading. His teacher would not let him read the books the older kids in class, who were in kindergarten, were reading. This turned into a daily power struggle between them wlth my son misbehaving every time she tried to force a phonics workbook on him. She insisted he must follow the curriculum, regardless of proficiency, or there would be nothing left for him to do. How very Montessori is that? In hindsight I should have pulled him out of there right away. My son also gave up on reading until he started kindergarten.
