It was horrible. I wasn't expected to do anything. Most days were spent bored to tears.
There was no oversight of the teachers; in fact the principal herself was the "teacher" for grades 1-3.
We were expected to keep a journal of our activities and show it to a teacher at the end of each day, who would sign it and let us go home. (The point was to encourage us to do things, rather than nothing.) There were two teachers on duty each day (the principal and her assistant) so I would simply lie; I wrote "did X at 9:34AM" "did Y at 10:53AM" etc then forged the assistant teacher's signature and showed it to the principal, who dutifully approved it (or vice-versa). Since there were about 60 kids and 2 teachers, there was no chance of them noticing.
A classmate intentionally smacked me in the head with the edge of a metal shovel and got off with a light warning, even after my parents came to explain the severity of the situation.
The activities that you could do in 1st grade were exactly the same as those you could do in 3rd grade. The next "transition" (room with different activities) was grade 4.
In my experience, the purpose of this particular Montessori school was to be a daycare, not to educate.
It was just awful. I'm not saying all Montessori schools are awful; I'm saying the one I was forced to endure was awful.