Thanks for all your replies guys. I passed really well today (wearing a suit and a new haircut helped) but in the end I did tell them, but only because I had to...
Apparently the school is big on swimming lessons as a confidence booster and physio help to the disabled children, and they need the staff to participate. They asked me if I would have a problem with that, so I decided to tell them - well, I'm trans, so I would need a private place to change and be allowed to wear a rash vest. If they want me for the job, I'd have to fill in a medical questionnaire anyway, so I thought it was better to be honest at that point. So, they know, and were very understanding about it. I hope my honesty won't lose me the chance of getting the job, but hey - I am more than just trans, so hopefully they will see it like that too.
Harry x