Not surprisingly, therefore, she is less than enthusiastic about the growth of educational MBA courses. "I was horrified when they appeared - what you have is not really an MBA at all but a management MEd."
She speaks from experience, having done a London Business School MBA over three years of part-time study in the late Eighties. "I was fascinated by the London Business School which had a part-time course designed to attract people from the public sector."
There was an enormous commitment, but even then, with the 1988 Education Reform Act still a year away, she was very aware, as a deputy head with headship in her sights, of the need to prepare herself for the coming changes. At the practical level, having the MBA certainly helped her career. Whether or not it got her the Holland Park job, only a few months after the course finished she became head of Queens' in 1990. "It certainly helped me to get on the short list, particularly as a woman with a career break in my CV.
"The course was enormously enriching. I was personally and intellectually challenged and made to cope with some things that for the first time in my life I found really difficult."
It improved her management skills. "Being exposed to finance and accounts and marketing and human resource management in a very rigorous way affects the whole way you think about problems and respond to them."
All the time, though, she returns to the benefits of doing the course with people from outside education. "The best thing was the network of people - there were 60 in the class, from the most enormous diversity of backgrounds - public sector, private sector, voluntary, and some funding themselves."
Never, though, did she think that doing a generalist MBA might provide a route out of school. "The one thing that made me cross was the number of people who assumed I wanted to get out of education. It showed the most extraordinary misunderstanding of what management in schools really is."
Mary Marsh still talks about her MBA with undimmed enthusiasm and stays in touch with other course members. "I've got friends I can phone to discuss particular management issues - they might be PR specialists, or able to give financial or legal advice."
She is also an alumnus of the School, and chairs a public-sector special interest group - reflecting her fear that as support funding has decreased it has become harder for heads, and other public-sector workers, to follow in her footsteps. "There should be bursaries for people from education. It would be enormously valuable if more public-sector managers were able to have that kind of training."