The big disadvantage with leaving school at 16 is that you won't be able to get a job that makes much money. It takes education in today's world to get a job that will allow you to buy a house, buy a car, go on trips, buy nice things. So, if you want an easier life stay in school. It is true that the more you learn the more you earn.
Sometime it is good for young men to leave school at 16, because you have more time to enter the university before you are recruited to the army.
Staying on in school untill you are 18 rises your educational level, provides the opportunity to aquire more skills and therefore more options in future.
Unfortunately just being at school doesn't guarantee that a student is learning. Those who achieve the best exam results will still be the most employable.
I suppose that it's good to stay at school until you are 18. School protects shildhood from some of the pressures in life. School can make children better prepared for adult life.