Lack of planning is the problem. On a cost per square foot basis, temporary classrooms are very close to the same as new construction. The primary difference is lead time. A new construction project can take a couple of years, just for the construction, disregarding whatever political process leads up to the school board deciding to pull the trigger. On the other hand, temporary classrooms can be set up in a few weeks. My school district has done plenty of both over the last several years, including both new schools at new sites and expansions to existing schools. New construction has decreased the need for trailer parks at schools, but not eliminated it.