Free is a good price, so I'd go with Autodesk Fusion 360. It's free until you make $100K a year on it, unless you're like me and generate too much data (
). It has reasonably decent toolpaths that are improving, and there's even a post-processor for the Shark(s?). Plus, as a post-processor, it's a modern-day programming-fool's paradise, far more capable then other post environments. I don't use their CAM output, but it has the whole thing, right there: even with articulating parts, components, and stress analysis. Even if you don't choose it, definitely check it out. I take models out of Fusion as STEP files to go to CAM for machining, or STL files for 3D printing. Around these parts, there are many people who feel Aspire and those products are a good investment for CAM-focused design programs, having a good balance of ease-of-use for design, and simple CAM options. Heck, I use BobCAD/CAM for the toolpaths, but absolutely abhor their drawing environment. Thus the importation of STEP models. But, that comes at a cost: for any major project, I dedicate a full work day to just validating cutting and other parameters. But, some of my parts also have spots needing +/- 0.002" tolerances, and I'm a big fan of shiny metal sculpted surfaces
But, here's the thing. If you REALLY want to teach the fundamentals of 3D modeling, the ONLY place to start is with OpenSCAD. Why? Because it teaches the fundamental concept that Constructive Solid Geometry is nothing but a hierarchical assembly of geometrical operations on simple geometric primitives in 2 and 3D, projected onto surfaces and all that other amazing math stuff. Do it. Understanding how OpenSCAD works will fundamentally improve how your students move forward. Understanding how to resolve the underlying complexities of that mental model will help them immeasurably in not only seeing how other tools work, but in knowing how to diagnose problems that arise. They'll be the brain AND the hero!
Hope that helps,
Thom