RE: Starboard equivalent for HiFly Mambo
Hmmmmm... how many days (hours) on the water for them to reach this level?
Most "A Taste of Windsurfing" students can beachstart by the end of their 2nd session. (1.5 - 2 hours on the water).
Perhaps have them "take a little test" before you put them on the GO boards?
Have them sail the Starts with the centerboard up, and with a larger rig to prove to your instructors that they can stay upwind without the center board. If they pass the "test" then they can sail the GO boards.
And, just having them sail the Starts without the centerboard, and with a slightly larger rig, should be an "upgrade" by itself.
Hope this helps,