Teachers bringing students in a group are responsible for the behaviour of those students, so it makes sense for them to accompany them in the cinema screen.
For GCSE workshops, we do require that teachers\an adult accompany their students in the cinema screen throughout the workshop. It really helps students to focus on the session content and, of course, also helps us maintain a high standard of behaviour. We would request that GCSE students are also supervised during the lunch break.
For A-Level and BTEC National workshops, there is no requirement for teachers to be with their students throughout the day. However, the vast majority of teachers do so, and this really helps their students make the most of the day.