Speaking as a student at a private university, I think one possible reason for the homework stressing them out is that they don't know how to do it. This wasn't a problem for me at my public high school, when the work was relatively easy, but now I'm finding myself stumped more often while doing assignments. Like, I'm a computer science major, and sometimes it takes hours to find out why my program keeps crashing, especially while working in C. Or even while working a math/physics problem, I'll just be pullin' my hair out when I can't figure out how to solve it.
The simplest solution would be for the students to go to the teachers for help when they need it, or to a tutor, or even to their parents or peers. But teachers aren't always available, tutors cost money, and your parents probably won't be able to help you unless they have, at the very least, a high school education. In those cases...I dunno.
Then again, I've never seen these kids' workloads. It's possible that they actually do have ungodly amounts of homework.
Or the little buggers could be simultaneously browsing Facebook or Reddit while doing their homework, and adding that time in with the time they reported to the researchers...