Both of my kids have now secured spots at highly selective overseas universities after completing their pre-tertiary education in Singapore (one in local and the other in International School). My oldest applied to 16 universities in three continents. She was rejected by 5, but got into her first choice school, which had a 9% acceptance rate.
Since there were so many applications, I could see how the differences in the details skewed the results. For example, the UK UCAS and U.S. common app had direct interfaces with the junior college she attended, and prompted them for updates. That left us with little control over what the colleges were receiving.
Anyone who has attended JC in Singapore knows that midyear & prelim in JC2 often see grades plummet which are often harder and not indicative of how they will fare on the A-level exam. Unfortunately those got sent to most of the schools, and I can 100% confirm: the Singapore context is not understood, despite what they say.
For the UC app, it was possible to better control the narrative and only provide final results without midyear or prelim scores. However, the UC app is tricky and uses rigid A-G categories and expects applicants (including overseas) to fill each one and it must be from classes taken in your last 4 years (S3-JC2). This is tough because history (A) is taken in S1-2, so what we did is use geography, which is a gray area, but it worked. For arts (F) it was also a challenge, so we added a home school that overlapped and added piano lessons, which were simple pass/fail ABRSM certificates. By positioning the application in the best possible way, an acceptance letter was received from every UC.
As for our youngest, he really didn’t have much of a chance at the most selective universities, so we took a totally different approach by going through community college and going via transfer guarantee. This option is available to locals and foreigners alike.
There are more insights I could provide, but I will stop here and see if there is any interest.