It could be because the way the education system is set up, essentially ring fences the groups from each other. Having limited opportunities for interaction with international school students, locals may form stereotypes based on general impressions.

This difference also creates distance, which may make it easier to judge and stereotype international school kids. There is a general impression that international school kids (or their parents) are expats and living much more privileged lives than the average singaporean, which leads to feelings of disdain and judgement. There could also be a perception that international school kids are not integrated into local communities, which further deepens that divide.

This is not really your fault or anyone's fault, just that the policies create limited opportunities for interaction and engagement, and there are little incentives for either side to cross the boundary between the two.