When you attend an interview for a job try and get a clear statement on what you are likely to be expected to teach. For example, if a job is advertised as a Chemistry post, will you be expected to teach some Combined Science or even some Physics and/or Biology also? If so, what proportion of your timetable will be devoted to your main subject? Staffing requirements do change each year, based on factors such as pupils' GCSE and 'A' level choices and staff promotions, and so no teacher can expect to have exactly the same timetable year after year. It is reasonable to expect you to teach subjects related to your main specialism, but it isn't reasonable to expect you to teach subjects for which you are clearly ill equipped. This would result in a bad experience both for you and your pupils. In my own career, I have always been happy to teach across all Humanities subjects, even though I trained to be a History teacher. However, I was once asked to consider being timetabled to take a lower school Maths class for a year, and I resisted this very strongly. I didn't feel my own understanding of Maths was such that I was capable of teaching it well. It simply wouldn't have worked. Most head teachers are reasonable people, and wouldn't want to put teachers into situations that would be bad for all concerned. If you find you have been asked to teach something you really don't feel capable of, then discuss matters straight away with your induction tutor. Of course, there will be lots of new things you will be teaching related to your main subject. This is to be expected and these new topics should be treated as a stimulating challenge.