>In those situations the Montessori classroom's weakness is that there is only one guide for all subjects as opposed to a traditional school's subject-specific teachers.

This isn't a hardset rule. We had the main teacher but we also had specific teachers as well for stuff like music, art, languages, or gym class. By middleschool there was no more "main" teacher. You were basically in a committee of teachers all specific including science, english, and history by that point. Part of that I'm sure was to prepare you for highschool in a non montessori setting.