DEFO is clearly the best term. The FLEX can only play defense, so why wouldn't they want to say DEFense Only?
Maybe NFHS can stop the presses and get rid of FLEX. If NCAA can change from "tenth player" to either of the others, why can't they all say the same? I GUARANTEE some coaches will think FLEX means they can also bat for someone else (or play linebacker).
While they are at it, how about putting both players in the same lineup card spot?