There are great tutorials on youtube. I found a lot of VEX based ones when we were starting up on CAD.
At the moment the design doesn't matter as much as learning the interface of Inventor.
You and your students should join the academic community at Autodesk. This will allow you to download just about any of their products for educational use for free..
Gives access to forums and tutorials too.
http://www.autodesk.com/education/home
It is how our team got started.. downloading the CAD files of the earlier years Kit of Parts (Vex and FRC) and used the youtube tutorials..
Sample Vex tutorial
https://www.youtube.com/watch?v=C-vSc79XsO8
(not associated with poster)
Follow a few, they get better.. But from a few of these you will start to understand it.
There are tons more but at the moment I cannot easily access them from school..
Good luck with it. Lots of fun!
Aloha