The head is not obliged to provide a confidential reference, although, under section 34.9 of the School Teachers' Pay and Conditions Document, he has a duty to provide information about work and performance where this is relevant to the teacher's future employment.
It is a little surprising that she gave her head's name as a referee, without first checking with him. As to the question of commitment, there is no contractual basis for this assertion in the normal teacher's conditions of employment. It is, I suspect, quite common for interviewees to be asked how long they envisage staying, and for them to hazard a guess. But this has no contractual force. A teacher may feel justified in leaving early, depending on how the job has worked out.