There are lots of 3D Modeling (CAD) software options. My preference are the free-ish versions so that students can continue to use the software after they graduate.
CAD Software (Free):
Fusion 360 by Autodesk – Preferred for mechanical CAD, blueprints, rendering, light STL modification. Register with your educational e-mail to maximize the features available. The free version is functional but has limitations.
Blender – Preferred for art, sculpting, and STL file manipulation
Utility Programs:
Meshmixer – Older, but reliable software for fixing and manipulating meshes.