Congratulations on getting as far as the interview, OP - they must have seen something they liked!
I take part in lots of interview panels at work. I don't work in a school, but some of the principles are similar, I think, in that an interview panel wants to find out who will be most able to do the job out of the people they're speaking to. So think about each of the components of the job, and why you both want to do them, and would be good at them. So a common question is 'Why do you want this job?'. When answering that, think about matching your skills, knowledge and inclinations to the job demands. I'd be looking for answers like:
'I want to work in a school context because I enjoy being around young people, and I want to be part of helping them make the most of themselves in exams'
'I'm very good at keeping calm under pressure, so I'm looking for a job that enables me to draw on that skill to reassure pupils and teachers when they're in the potentially stressful exam situation'
'I'm extremely well-organised, but also people-focused, and I think this job would enable me to make the most of both because it demands meticulous organisation alongside working with young people who may at times be unpredictable'
NB these might not be the perfect march for the job itself, I'm just guessing the sorts of things they might want. You could look at the job description and person spec for more ideas. Key is, whatever the question, keep telling them why you would be good at the job and why you would like it (they want someone who'll be happy and is therefore likely to stay).
I'm always impressed at the end if someone asks about training opportunities (it shows they want to do a good job and keep their skills updated), and if they ask a question that shows they've done their research about the setting they want to work in. Is there anything you can think of to ask about the school itself, or the way it organises exams which would show you've been finding out about it? E.g. 'I'm excited about the prospect of working in School A because I think it might make some adjustments to exams for pupils' special needs. Can you tell me if I'm likely to be involved in this aspect?"
I'm less impressed if someone asks about things that were clearly stated on the information they've been sent, particularly holiday entitlement! But if there's anything unclear on the info you've been sent, you can ask about that.
Good luck with the interview.