CDs and pencils seem like the ideal tire-and-axle for a low budget vehicle challenge, but sticking them together is a nightmare...unless you print the rims.

I tried starting with a circle and building up - FreeCAD didn't like that.
I tried sketching a cross-section and rotating - that worked for two wedges, but it won't paste or map onto the raw edges without arbitrarily mirroring, and there's no option to flip it back.
I tried sketching the cross-section out 12 times - that crashed.
So here's the two wedges copied and cobbled together. I can't wait for FreeCAD to get a working Assembly workbench...

There are a few problems with the design, but you get the idea. Making your own is half the fun!

Students will learn the basic laws of physics - drag, thrust, wind resistance, etc. - on top of learning 3d modeling

High school or junior high school students would be ideal - customized circular models will be vital to success

Students should already know how to design and print basic 3D models; teachers should have limitations decided in advance (can they 3d print the entire car? Are they allowed to download designs from the Thingiverse? How long do they have? Can they work at home? Can their parents help?)

• Students should form (or be formed into) groups.
• Students design their vehicles - frame, wheels, and motor. Everyone using rubber band power or everyone using different power sources can be good.
• Students model and print the missing bits - drinking straw connectors, pencil hubs, clothespin axles, etc.
• The groups then race in a large indoor space (the gym? Cafeteria? A hall?) - prizes are awarded for the car that gets the best time and the car that goes the furthest on onboard power.

Each groups will have made a vehicle; grades can be based either on the performance of the group vehicle, or on the effort put into construction.