Like many things in a Montessori school, it varies- that's the true answer
I taught drama in a M school for four years. This is my first year at a new school and I'm teaching music now. I can tell you that the use of theatre varies widely depending on the teachers and on the head of school. Ideally, I think that theatre would be integrated into the classroom just like visual art, music or any other type of "special" area should be. Practically, however, most teachers do not feel that they have strengths in all of these areas and some school as well feel that they would like to have "specialists" come in to teach these subjects.
When I was teaching, we had classes that the children came to in a classroom outside of their own and I created a curriculum that tied into things that the children were doing in the classroom as well as giving the children a lot of artistic license but exposing them to the basics of theatre studies, acting, improv, etc. I worked closely with the art and music teachers as well and it all seemed to be a great relationship.
Well, then we got a new head of school. One who wasn't Montessori trained but thought he knew more than those who were- that type
- and he was "quietly" against having specialists in general, thought he finally came out and told us this. So, to make a long story short, I ended up teaching more music than drama, was told we shouldn't have any school-wide performance and was then left to tell the kids every day when they asked if we could do a musical like we did last year that we weren't allowed to. Ironically, the head of school eliminated these performances because he didn't want "adult run" performances- he wanted all things dramatic to be complete products from the kids which sounds great, but it doesn't happen. It's like hoping that a child will do long division one day without ever being taught math. They were aching for me to work on a play with them. Unfortunately, the children don't have any drama anymore, no more performances- though my classes were more focused on the process and not the product- and have very little time for fostering dramatic play in the classroom. They miss it terribly.
All in all, I'd say it totally depends on the school. I've seen schools that pride themselves on all the dramatic performances that they do and schools like the one I left where they've chosen to deemphasize it dramatically. Look for one that has a nice balance- I think it's important! And the kids love it. I feel really bad for my old students and I miss it myself.