Because your test doesn't prove ID wrong. It would only demonstrate that an alternative method was at work, but there is not requirement that if ID is true no other process could be as well. They aren't exclusionary. Later evolution could have been "designed" in.
Until we have a body of knowledge in this area, and a way of applying it to problems of natural history, the ID speculations have no place in the elementary science curriculum because they're without content.
Your statement is self-contradictory and silly. If it can be proven that a set of characteristics which work together gradually evolved then ID is refuted, period. Evolution cannot "design" anything, it is a mindless force, it has no intelligence, no memory, no brain. Whatever it does, however it does it, cannot be called design.