Typical questions that might appear in the interview:
What made you apply for this role?
What skills do you think you bring to the job?
What experience do you have of administrative work?
What do you know about the school?
How familiar are you with word and excel (and maybe powerpoint)?
How do you prioritise?
What have you done during your careers break)?
What are your strengths/weaknesses?
How have you kept up to date with latest technology etc?
I have interviewed loads of people as a senior manager (and I am also a professional careers consultant) and it is a complete truism that people form their opinions in the first minute. So it is important to be enthusiastic and confident from the start.
Can I also say, that even though this is a school try to think of work related examples during your career break. Unfortunately the multitude of skills we use when managing the family are not that highly valued be employers (even if they damn well should be). So have you been on a school committee, run a playgroup, taken a course, done any charity stuff that you can use? It's good to mention what waynetta said but you need some vocational examples as well.
My advice, do a few sample interviews with family before the real one. Get them to throw loads of questions at you so you don't get caught out and it'll also build up your confidence. Make sure your answers use tangible examples that stick in their heads.
So: How do you prioritise? I keep a diary.
Instead something like... "Well each morning I look through my list of tasks and highlight urgent ones. I deal with these first then systematically go through my in-tray ensuring that nothing has been in there longer than a day. If a task is given to me during the day and I think I am unable to make the deadline, I ensure that I inform the person in good time to discuss whether it can be delayed, if not I reprioritise etc etc."
I know it's blindingly obvious but it shows you have a system and have thought things through.
The truth is that I doubt the interview will be very rigorous. What you might also want to do is to learn how to do a spreadsheet (they are a doddle) and practise loads on your computer before Thursday. Good luck.