The merle's whisker reagent (taken from Merle's own beard haha) is classed as 'any school' and since it must contain his dna / essence it suggests he is also 'any school'.
"I was apprenticed to a program long, long ago that fostered a cross-discipline approach to Wizardry. At various times I have spent a few centuries devoting myself to the study of each of the different magic schools. Currently I am studying the Astral magics, a school of thought that is barely recognized and little known.
You see, at Ravenwood, we teach the core essentials of both the Inner and the Outer triangles of Wizardry: the External magics, which deal with manipulating the forces of Nature (Fire, Ice and Storm) and the Internal magics, which deal with the elements that compose one's self (Mind, Body and Spirit ... or, as you know them, Myth, Death and Life.) I am currently researching a third triangle, one that we know little about, but seems to be quite puzzling and immensely powerful; the Astral magics. These deal with the manipulation of the universe itself, the Sun, the Moon and the Stars; those heavenly bodies which are bound by (and make up) the Spiral itself, the inner workings of the great machine which makes the Universe tick... much like an old (but perfectly maintained) timepiece.
Oh, my! That was a bit long winded, and I seem to have lost my train of thought. What was your question, again?"
Well, in the earliest draft of the story, Greyrose was the headmistress and Merle was the ice teacher. I think that's as close to an answer as we're going to get.
