You could investigate how the school has changed architecturally over time and how this has affected the design of the classrooms or the structure of the institution. You could look at whether there are differences between all-girls and all-boys schools or mixed schools and/or relate this to different country contexts as well.
Take time and browse research works on gender in educational practices and class room environment using "Google scholar" and other search engines. You will be able to decide the topic based on previous works underpinning your research gaps. The more you read, the more you will be able to decide well focused and specific PhD research topic. Best wishes!!