let me be more constructive.
i've been a tutor to upper secondary students for the past 6 years, myself having gone through 2 years of JC life and 4 years of university life, while a large segment of my social circle comes from the polytechnics.
before i go on let me cite you a CASE STUDY:
My cousin is retaking his A levels this year, this is his 3rd year in JC. he got into this JC appealing through sports, his O levels he scored 17 points.
The JC (and many JC's nowadays) dictated what subjects he must take. Couldn't take 4 subjects, and the 3 subjects he was forced to take were Econs, Chemistry, Maths. The school's stand is "take this subject combi, or go elsewhere to another JC".
(It is my belief that the school forces students to take these subjects because their track record in these subjects are stronger, and their school ranking goes up. Contrast to allowing students to take the subjects they love, but its more difficult and pull the school rankings down.)
His biggest regret is not taking Physics because he wanted to do engineering in university. He stuck to the JC, struggled and wondered why he even came to JC doing subjects that wouldn lead him to the path he wants to go. He was completely unmotivated.
Here's the catch; he stuck to that JC because he FELT that going to a polytechnic was inferior. all his friends were going to good JC's (again an issue of ranking) and he followed the paper chase.
Sadly, even if he does well this year for his A levels, he still can't do MAE at NTU, the path he wanted to take. Even if he could, he has a lot of catching up to do.
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Ironically i came from the same JC as my cousin, only 7 years ago. Back then, they allowed me to take 4 subjects even though i didn't qualify. They held rock concerts for school founder's day. The school ranking was the pits but everyone loved what they were studying, and still majority made it to wherever they intended to go.
After a change in principal, they decided to want to "top the charts". No more D's or C's shall be tolerated at A levels. Now they bar students from taking A levels if they did badly from their prelims!!!
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i know my disdain for the JC system is overly apparent, but that's because of the realisations that i got after the pieces of information came together from so many sources and years as an observer in this "industry".
My advice to those considering going to JC: Go to a JC that is less interested in ranking, like those not in the top 10. ask around from your seniors and find out which JCs have students that are happy and which JCs suffocate their students. Do not just consider school's ranking.