Tinkercad is a free, web based, CAD app that runs in your browser (need webGL enabled). It's easy to use and has some great features. Many CAD systems that engineers use are very complicated with thousands of features (I've used CATIA, ProEngineer, AutoCad, Cadkey) that students won't need.
Tinkercad allows you to create 3D models of pretty much anything, and even export them to 3D printers. The site has a getting started page that walks you through how to use it.
There is also a community of users for support and help.
This is a great resource for students and teachers to use in STEM Classes, or any class for that matter, to draw 3D models.
Google Sells SketchUp 3D modeling software - promises free educational version to continue
SketchUp for Education Site
Google Resources for Education
STEM Resources for Students and Educators
LibreCAD - free, 2D CAD Software for Windows, OSX, and Linux
Free Academic Program (incl CAD software) from PTC