At the same time as Mrs Gumley Mason sent the letter to all parents, another letter was sent to all staff, and a copy has been made available to me.
This letter is not much more informative than the letter to parents. No reason is given for her departure. The one additional piece of information provided is that she states that she will be handing over to Mrs Wilson, who will become Acting Headmistress with effect from 1 January 2012.
I have to say that it is most unusual for a headteacher to leave part way through the academic year. And it is even more unusual (except where the departure is necessitated by health reasons) for a headteacher to cause an interregnum by leaving before a permanent successor has been appointed and is ready to take up his or her duties. I do hope Mrs Gumley Mason is not suffering ill health - I would not wish that on anybody.
It would appear that the post has not yet even been advertised. I've just made a search of the job listings on the Times Education Supplement website (where previous vacancies at the school have been advertised) and the vacancy is not yet listed there.
I think parents (who are paying for their daughters to attend) do deserve a bit more explanation than has so far been forthcoming. It is far from ideal to leave a school with an acting headteacher. I know nothing of Mrs Wilson, I have no reason to think that she will do anything other than her very best in the post. The problem is not with the person who will be acting headteacher, but rather the disruption that will inevitably occur as a result of two changes of headteacher in quick succession.