Ooh Wreck so would that be term time only? I can see why that would appeal.
I think I remember what you do now - is the school job more hands-on and not just clerical? If I were interviewing I would be keen to know why you wanted the job, and also would be pleased if you knew a bit about the school and had a positive reason to want to work there. It's perfectly fair enough to be interested in a job because the hours fit (and that is probably what would interest a lot of people!) so you would stand out more if you could explain why it is a natural progression from your current role and why you would be good for it (you might, for instance, be more than used to clinical record keeping, understanding confidentiality requirements, pretty confident with the language of healthcare, able to deal with people who are scared /difficult... you might enjoy the contact with children most in your role, whatever). If the job involves new skills and responsibilities that you haven't experienced don't be afraid to admit that you would need some training, but turn that into a positive and explain that that is why you are so interested in the role - because you will learn something new! I don't know much about school health assistants but I imagine you will need to be able to work fairly independently, but know the limits of your
Knowledge and use good judgment and know when to escalate an issue appropriately. (Basically, let them know you have good common sense and can get on with things day-to-day but know when to involve others).
A question I would ask if I were interviewing people for a role like that is about safeguarding. You've probably already had training on that and so should be able to handle a 'what would you do in this scenario?' -type question but worth refreshing your memory and thinking that through a bit. If you are concerned about a child how might you handle it? What sort of signs should you be concerned about?
Another good thing would be to know a bit about the school, and to use that to demonstrate why it would be an interesting role for you. I wouldn't go too heavy quoting statistics etc (that comes across a bit weird!) but if you said, for instance, that you are aware from the ofsted report that the school has quite a broad socioeconomic mix (e.g. as shown by higher than average free school meals) that you would expect a range of issues to impact on health, making it a challenging but interesting place to work etc etc - you get the idea!. You could see from the intro to the latest ofsted report whether the school is above or below national average for FSM or number of languages spoken. If above, that will mean it has quite a socioeconomic mix (with attendant issues). If below, say in a very middle class affluent area, it might suggest that the pupils have more home advantages and that you will have a quiet life but you can still comment on it! For instance you would be able to note that it is a nice school in a nice area but that there are always issues impacting on child health that would make it an interesting role (if they ask you to expand you could explain that for instance obesity is not predominantly a class issue; likewise emotional and physical abuse can happen anywhere, and in a less dramatic level family pressure and expectations can really affect children's health too). I am sure in your current role you have seen that things are not always as they first seem so you have to be interested in people and open minded to pick up on the signals of what people need.
Finally, they will probably ask you if you have any questions. Worth thinking of one if two good ones - anything you genuinely need to know is fair enough, but also something about the role (how it would be supported /who is the manager, training opportunities, opportunities for you to liaise with staff across the whole school to build you working relationships... or opportunities to meet school health assistants from other schools to learn from each other... whatever)
I hope I haven't overstepped the mark with this essay, I am just interested and know how few people do this little bit of extra prep!
Perhaps I just need to get out and get back to work 
I obviously miss it!