Trailers can be moved to accommodate chances in population easier than brick buildings. They come in several styles from dumpy to quite posh. Much like regular construction. They can be kept up or let get junky just like regular construction. Here in California they made more noise during a earthquake. When the F4s landed or took off they were louder. The pilots came back about a day before the carriers did usually. They were also closer to the playground witch was good to get to the tether ball pole first. The worst thing about was the lower ceiling made it harder to throw a ball while the teacher’s back was turned.
This is exactly why they had trailers in my schools. My part of town was a new expanding area, with new neighborhoods, which are exactly the neighborhoods which attract families with small children starting school. The school district built the main school buildings plus added modular buildings for the boom years. After we all graduated high school, the demographic for school age kids plummeted, and they could remove the modulars at the schools. This was just good demographic planning on the school district's part.