I'm not sure if all schools define big/small bb the same way. But u guess if bb reaches 3 when starting K1 school on 1 Sept (or similar case when reaching 2 when PN starts in 1 Sept), the bb is to be considered as big bb. This is the case as in Vic where has big bb born from 1 Jan to 31 Aug and small bb if born on 1 Sept to 31 Dec.
I may be wrong as different schools may their own definition. However, some parents may think this sort of system is unfair since some small bb can perform
as good as big bb.