A: There are two issues here. Firstly, how often is too often when moving schools and, secondly, should I go for promotion to head of department after six years in teaching? The answer to the second question is yes, why not, if you can prove you can do the job.
I would be more concerned if you saw this post as the opportunity to get away from a school where you were unhappy. As to the first part, there is less of a clear-cut answer. If you are someone whose partner's job requires frequent moves, then you have little alternative but to move with them. This is a perfectly legitimate reason and most schools should understand. However, do you then take the first job that you can or wait for something suitable? I guess the degree of compromise will depend upon the frequency that jobs appear in the area.
Sometimes we all make mistakes by taking jobs that seem ideal at the time, but later prove to be a disaster. I know, this has happened to me. Two years is quite long enough to have stuck with a job in a school where you feel unhappy. Even if you weren't considering promotion, it would be time to look for another post where you could feel more comfortable. You will be a better teacher and less stressed.
John Howson is a recruitment analyst and visiting professor of education at Oxford Brookes University. To ask him a question, email him at firstname.lastname@example.org.