We cannot answer this question in single terms because the definition of Tough or Toughest may vary. It, actually, depends how we define “Tough”, “Hard”, “Good”, Easy and so on. It is determined by on how you want to calculate a certain standard of progress. It is very difficult to compare the education systems in the various countries. It’s hard to interpret the toughest education system.
Children who study in India may feel it as the toughest to learn maths, trigonometry and science.
If we compare in terms of cost, we may feel that the US is the toughest country to live in as many cannot afford the cost.
A few students say France has the toughest Mathematics. Math is not only an important subject but useful to ascend the ranks in Government and private sector jobs.
Some students say Indonesia's education system is worse than mediocre. It is divided into three main levels which include SD which is equivalent to 6 years of school, SMP junior high school which is there for 3 years and SMA is the senior high school for 3 years.
The school in Korea used to be very exhausting but I think is quickly changing.
Yes, the concept of unnecessarily stretched classes and the after-school classes at school have overburdened students. However, there are schools on this planet that teach you how to think and find the origin of information rather than forcing you to memorize stuff.