Well, I dunno, man. I mean, it really depends on the school and the individual students. From my experience, the kids in my base school are happy. They've got cheerful dispositions and are generally quite open and friendly. The kids at the technical school are...different, but I don't see them as being particularly stressed either (of course, none of them are going to university afterwards, so that's probably part of the reason for the lack fo stress).
I imagine you would get a far different answer if you asked someone who taught at a SELHi (Super English Language high school) or a top-ranked school.
EDIT: Synthesizer Patel taught at a high-level high school in Japan, he could probably give a better answer than me.