It is against policy for movie theater attendees to wear masks to theaters, so we still don’t know if management was aware of the costume when the teens arrived at the theater.
The attendees of the Mission: Impossible – Fallout screening were understandably upset about the situation, and later accused the management at the theater of being part of the joke.
However, video taken during the aftermath of the ordeal seemed to show workers at the cinema explaining that they had told the teens to wait outside of the theater for the next showing that would take place in that auditorium — a screening of Slender Man, which would explain the costume.