My daughter is on 油蔴地天主教小學 P.3. She has seen 插班人 on both P.2 and P.3.
The P.3 one is making a good friend to my daughter so I know his story better. My daughter told me that the boy was No.1 in his last school. Now he is among the top 3 in her class.
I have asked many times which school the boy came from, sadly my daughter never bother to ask the question.
Generally speaking, those band 1 schools have tens of people knocking on their doors every year competing for a handful of empty seats; so the shcools have lots of choices. So if your kid has not much eye-catching things, you may just save the hassles and spend the efforts to get him to excel in his current school.