I would say nothing is guaranteed but subject to the effort by your child and sometimes LUCK - especially working within such an allocation scheme!
My case is she could not be allocated to her favorite, a prestigious girls' school as the first choice but still a good Co-ed school (ranked thirtieth something in Khmama's latest list). She's then changed to HTC and got accepted shortly and now it sounds got excited awaiting a new page in secondary school life.
But she finds it unforgettable with her primary school life and care from KTS. Teachers and Principal are very nice and always concerned with the students there.