Hi, yes u can do that if you're (academically) gifted and won't mind the grind too much. There were a few transfer students at my SAP school, usually foreigners but a few locals too, sec 2 or 3, so not unusual. The students themselves were unusual though, very independent, some stayed on in the Singapore system and some went back overseas.
About the schools, you'll more or less have your pick if you've got the results to show for. Any awards or recognition you have in CCA/independent competitions will help. It's quite risky to try and enter at the Sec 3 level though; many schools will be concerned about their ranking/your ability to catch up and will monitor you after you get in (some gifted kids don't respond well to this style of handling) .. especially the SAPs coz, as u probably know, they tend to accelerate their curriculum and turn most of sec 4 into a revision year. Not very pleasant. Sometimes they may turn down transfers just because of internal reshuffle or resource concerns, or offer placement in lower sec coz they're scared abt rankings, so don't take it personal. The good non-SAP schools will depend on their management, they may try to jack up their entry requirements if they're chasing rank hard that year. Apart from that, u may or may not have to put attention to making friends/working in groups coz everyone will have less time and may already have their own cliques.
It's probably better to hold off for 1st year poly/JC if u can, it'll be easier and everyone will be starting afresh too.
If you're still keen on entering at sec school level w/o placehold, these are the requirements -
https://www.moe.gov.sg/admissions/retur ... ary-school
Also more on
Eligibility & cut-off
GL