Morning TV message
A lot of movies set in American schools include morning announcements from the principal, often via TV screens. While most schools have some sort of announcement system to let everyone know what’s going on, in nearly all English schools this is done gathered together in one large hall.
The thing is, often American schools are huge, with thousands of pupils, and getting everyone together in one room is impossible. Screening the Principal’s message into classrooms is a much more sensible use of time. So it’s not just a movie thing.