The plot of the "Three Little Pigs" requires a set that includes three houses -- one of straw, one of sticks and one of bricks. An effective stage set for this play will somehow convey to the audience that the houses physically go from unharmed to blown down. The transformation of the houses must be simple enough for the child actors to do onstage during the performance.
Constructing plywood sets is the most difficult and costly option for a school play. Build three sturdy wooden frames from 2-by-4s to support three large pieces of plywood, at least 4-by-6 feet. Paint houses on the plywood to resemble the straw, stick and brick homes of the pigs. When the wolf blows the house down, he will pull a sheet down over the structure. The sheet will be decorated with remnants of the building material, such as bits of straw or some twigs.
PVC pipes and fittings are extremely adaptable for use in a stage set. Consider making PVC pipe frames for each house that resemble standing rectangles. Use fabric paint to create the individual houses on long fabric pieces, with the appropriate details to resemble straw, sticks and bricks. Hang the fabric from the frame with shower curtain rings attached to the topmost pipe. Cut a window and door in the fabric for the actors to use. When the wolf blows the house down, the actor simply pushes the fabric aside, hiding it from the audience.
As long as there is a suggestion of what the scene should be, a minimalist set could work very well. Use three umbrellas each covered with straw, sticks or foam bricks to represent the little pig houses. The actors fold up the first two umbrella houses when the wolf huffs and puffs. Or, the actors could wear cardboard sandwich boards to represent the houses, then remove them as the houses are destroyed.