Whenever I teach Inventor, I start simple. First make a basic part that goes down the tool line, in both sketch and features. Have them all work through making this part, as you cover over it on a projector, or something they all can see. After doing several of those (I usually do 3 or so), hand them a drawing of a part, just as someone would during the build season. However, have one or two dimensions missing. Don't make it impossible by the lack of these dimensions, but make it so they have to do some basic math or geometry in order to figure it out. If that challenge doesn't sound good, then try handing them a hand drawing of a part that is dimensioned correctly, but drawn off scale. This will start preparing them for CADing parts that team members hand them, in order to aid the machinists.

Once they have the basics down, give them another challenge, this one free form. Hand them a pair of calipers (digital or not, doesn't matter), and tell them to pick some machined part from last year's robot. Have them record all dimensions, and go CAD it up. Once they finish, send them on to a more complex part. This set of challenges is nice because it allows them to scale their CAD to their skillset at the time. So if one kid is getting it faster, you can give him a more complex part.

If you want to throw them a brain bender, have them CAD a part with a hole that is at 45 degrees to the face it's drilled into. That should stump them for a week or three.

Have fun teaching!