First, just FYI: A pre-ban collapsable stock fully extended is "A1" length; i.e., the same length as the original M16 and M16A1 fixed stock.  The current stock (the "A2") is 5/8" LONGER.

Anyway, you could conceivably buy a collapsable stock, fix it in position 2 or 3, epoxy in place, then add at least 4 non-removable blind pins (2 per side).  This SHOULD leave you with a stock that BATF would consider "fixed", based on their previous approvals.  You could then attach the stock to the rifle.

Of course, it might then be too short for the guys.  A possible solution might be to get one of the rubber extended "recoil pads" made for the collapsable stock and use that when the guys shoot the gun, and remove it (about 1" in length) when the ladies shoot.

It isn't at all practical to swap stocks out with any regularity, just FYI.

You might also note that US soldiers of all sizes and genders learn on A2-stocked M16s.  The fact is that most people don't use the proper hold/stance, which is part of the problem.  I would suggest looking for a used, fixed A1 stock (usually $10-15) and see if that isn't acceptable.  It would be a heck of a lot less work, and would leave the gun with the more reliable "full-size" recoil system vs. the carbine system.

-Troy