Have it before applying, not after an offer arrives. Split the two questions — do you want this school, and do you want to do this for four to six years — and give a veto you actually intend to honour. Then agree, in advance, what happens if the answer is no.
Have it before the application, not after the offer. Once a confirmed offer is on the table in August, every conversation in the house is about a place that already exists, and a 12-year-old will read the family's hope accurately and answer accordingly.
Separate the two questions your child is actually being asked, because adults routinely merge them. The first is: do you want to go to this school? The second is: do you want to do this thing, at a serious level, for the next four to six years? A child can genuinely mean yes to one and no to the other, and only the second question is what the commitment binds them to.
Give a real veto and say out loud that it is real. If the child's no would not change the outcome, do not stage the conversation — it teaches them their answer is decorative. If it would, tell them so, and then honour it.
Talk about the rejection before it can happen. Applicants who receive no offer take part in the S1 Posting Exercise like everyone else, and nothing about a DSA-Sec outcome touches the PSLE. Saying that calmly in April is worth more than saying it in August to a disappointed child.
Finally, rehearse the honest answer to the ranking question, not a tactical one. Children are reportedly asked, in various forms, which school they would choose if two of them made an offer. Nothing in the public record establishes what any school does with the answer, and anyone who tells you otherwise is guessing. What is knowable is that a child who answers honestly can keep answering follow-ups, and a child who answers strategically usually cannot.