SketchUp and Cura are 2 apps that you can download for free. SketchUp is is a 3D modeling program for a wide range of drawing applications such as architectural, interior design, civil and mechanical engineering and Cura is developed by Ultimaker to make 3D Printing as easy and streamlined as possible. It includes everything you need to prepare a 3D file for printing and to print it.
Cleethorpes Academy have a great PDF for you to download that gives step by step instructions for preparing your SketchUp files to move them into Cura and print them.
In The Classroom
This resource make a good student handout as it provides a short step-by-step guide with screenshots to assist students in saving and importing their 3D models from SketchUp to Cura.