IB population is cherry picked from the top 15% or so of cohort from elementary school.
Many people have ridiculous tuition outside of school. Most of my mates had tuition which costed approx $6k USD per month, outside of school.
Those who cant afford the tuition see the difference in aptitude, and are compelled to study harder or get fucked.
Plus, the government does a good job in limiting salaries of most workers with very liberal foreign labour policy, while having strict protectionism for more elite fields like med, law and special engineering courses. This suppresses wages for all but the top 5% of a cohort, while bolstering the wages of the top 5%, which basically provides some healthy pressure to work harder. An average college grad will make about $1500 USD per month after graduation. A straight A student who will most likely than not get a government scholarship can get paid up to $S3000 per month while still in college with all college fees paid for.
Law graduates make about $S5000 after graduation, which is already more than the median salary of the labour force at $4300. My own parents had end career incomes of around the median, even with a masters degree from the local universities.
An average technical school graduate would be looking at about $900 USD per month, and their end career salaries are around $2000 to $3000. Thus, not getting into college is also a big pain in the ass.
Thus, the majority of rational students will see this, and know that they will get fucked hard if they dont get into a top college course, thus arguably making them work harder then they would if there were no such selection pressures.
From a governmental point of view, this kind of policy is a genius use of the free market to squeeze out every ounce of productivity out of the labour force, and thus attracting more investment. The standard of living is consequently lower for sure due to this, but most people seem happy to make these trade offs.