RE: Which *general* beginner board?
The Superlight was OK as a training board back in the day, but the older Starts (before they added the retractable centerboards) are really about the best.
I just bought an '02 Start and I won't be getting any Starts or Rio's next year until I see that they actually work as good as the older Start I just bought.
I like the older Starts with the removable center fin as you can change the fin to match sail size, weed conditions, etc.
Or put in the plug and go quite fast with a big rig in true shortboard mode.
What sails are you using for teaching.
That's the other HUGE advancement in entry level gear.
Lightweight powerful trainer sails in the 2.5-5.0 m2 range make teaching adults of all size so much easier and in less wind than you now think you need.
I spent this last weekend teaching in winds often < 4 knots.
You just have to adapt some techniques to make the boards go.
This weekend, was quite alot of "pushing" with one finger on the stern of the board to overcome the initial inertia, and then the students were able to sail and steer OK for about 50 ft at a time without needing any additioonal "push".
Hope this helps,