OK, I know this is a bit bizarre, but I was thinking about this the other day...
What is someone gave birth to twins, and one came out at 11.59pm on 31st August and the second at 12.07 on 1st September???
Would they have to be in different school years?
I suppose to widen the discussion, what about children (e.g. step children + birth children) who are only a few days apart, but in different school years?