I'm not clear from your description if you'd be doing the first aid or training others or both? So sorry if some of my examples below don't quite fit!
If they know you don't have any experience, I guess they are planning on training you up.
So they'll be looking for suitable skills that you can do the job once trained: perhaps communication skills, explaining information, keeping calm in a crisis, reassuring people who are anxious, developing rapport with teenagers, following procedures/being aware of health and safety, reliable etc
What is on the job description?
It's worth thinking of times you've shown those skills in the past in other roles.
It might be worth reading TES or BBC education section for a couple of weeks beforehand so you've got an idea of things going on in secondary schools at the moment.
Look at NHS website (or another reliable one) for first aid tips or get a couple of first aid books out the library. You'll feel more confident and you can tell them you've done that as an example of how you'd take the role seriously.
Have you got friends/family (or friends of friends) working in similar areas you could talk to?
If they ask you something you don't know, be honest but suggest how you'd tackle it.
"I don't know exactly the right thing to do in x situation so I would check the school policy/first aid procedure/NHS guidelines etc. A common sense approach would be xyz because the most important thing is keeping pupils safe and calm"
Good luck!