I feel hesitant recommending anything since everyone's situation is different. We went into the assessment without saying anything and the teacher did discover very quickly what DS was cabable of and she told us at the end of the assessment what she had planned, which was quite advanced work. I think it is nice for teachers to feel that they have discovered a prodigy and this approach set a good tone of collaboration. This teacher has been a great guide and mentor to him over the last 6 years.
But DS has never been one to hide his light under a bushel, so it never occured to me that he could not be discovered. Either a more reserved child or a less astute teacher might create misunderstanding. So I don't wholeheartedly recommend our approach to everyone.
I will say again that volunteering in DS's class for all these years (one morning a week) has been a fabulous opporunity to create warm relationships with the school. If this is at all possible, I would highly recommend it.