In the private sector it is very common to expect that references will be sought, but only after the recruiting company has selected you and made you an offer 'subject to references'. In that situation the interviewee has to disclose to his/her current employer that resignation is imminent.
In the public sector, and certainly in the teaching sector, there is much more openness about the employee going for another job and indeed amongst teachers it seems to be necessary that the existing Head knows because teachers cannot take a term day off as 'holiday', for which to attend an interview at another school.
I imagine you are currently working in the private sector and going for a non-teaching school job, in which case I think it is perfectly reasonable that you point out to the school that references will be available from your existing employer following your selection for the job.