Somebody said it earlier. The Oxford English Dictionary (for whom I used to work), accepts both forms but gives 'ize' preference in UK English. I have no idea how 'ise' crept into modern teaching in UK (and Australian) schools. The problem now is that Word believes that 'ise' is British.

Strictly, strictly speaking, educated writing everywhere should use 'ize' - but we have a changing situation where people think it's British to use 'ise'