I have to say that the pressure is put on the children in Beijing by the parents and not the education system. My DS is actually a teacher in Beijing and the parents actually try and pay him to make sure their children pass the exam. At a very young age they are encouraged to study hard and I have to say I think this is what they are used to. The kids are not stressed out by this form of education, this is their way of life and culture.
I do agree pollution levels can be really bad and that isn't good for anyone.
I agree a lot of kids have private tutors as well, my DS also does this ,but, I also have to say this is all done in a very friendly way and he is treated like a family member and they try and feed him all the time. I have seen pictures of his students and I must admit, I have never seen any of his students either in class or at private lessons, looking worn out. Although possibly there could be some schools where this may happen as Beijing is a very large city.