“School Rooms” or “Dog Boxes” are terms often used in the film industry to refer to the trailers where child actors receive their education while on set. Laws mandate that child actors continue their education during filming, so they will often have tutors or teachers on set. Also these can be used as costume, make-up or production vehicle. If there is any need for extra space on set the school rooms is a perfect vehicle for the job.
“Dog Boxes,” on the other hand, is a more casual, industry slang term. It’s used less frequently and its origins are unclear, though it might relate to the small, confined spaces where these on-set classes sometimes take place, similar to a dog’s crate or box.