It somewhat depends on what is being designed.
Most CAD type tools are going to rather easily make simple primitive shapes like blocks, spheres, cylinders and such.
If there is a desire to create more organic parts, then it is best tool look at software which allows for free-form modeling.
Give us an idea or images of what will be designed, and specific programs can be suggested.
3D printing can make a lot of things, but it is ideal for complex designs.
In our lab last week we had a visitor come by that wanted to use our 3D printer to make some parts. The parts were going to take 12 hours of build time. Looking at the parts, they were simply rectangular blocks with some holes. I was able to cut and drill four pieces of wood to the appropriate dimensions in just over an hour.
I only point this out because people have forgotten that things were made prior to 3D printers. Teach the kids some valuable hands on skills to cast something in plaster, shape some clay, or cut some wood. Not everything needs to be printed.