This project is currently being used for a middle school intro to engineering program. This project is pretty simple to build and just about any home improvement store will have the supplies.
The kids are given the materials and the trebuchet drawing (see link in description). It takes the average group (2 to 5 kids) about 10 minutes to assembly the trebuchet. There are two variables in the setup. The first is the launching arm – three choices are available: short, medium and long. The other variable is the height of the weight. This is controlled by the distance on the ruler attached on the board at the base of the trebuchet. Data is collected for the three arm lengths across the weight height range (see link to spreadsheet). After the data is collected, a competition is held to hit a target (bucket, 20 feet away).
Although modified for the class requirements, this is not an original design. It was reversed engineered from a combination of the versions you will see online.
See video for actual build.