Tinkercad is an amazingly powerful easy-to-use tool for creating digital designs that are ready to be 3D printed into super-cool physical objects. You will be guided through the 3D design process via easy hands-on "Lessons", that teach you the basics of Tinkercad before moving on to more complex modeling techniques.
Teachers and students just need to simply go to TinkerCAD.com and use the Google sign-in and their school email to create an account.