It is difficult to blame academic stress on homework alone. Homework, here, should be considered an extension of the academic pressure many high school students face when attempting to excel and succeed in the competition for higher education. I think your own personal experience and observation should be enough to tell you that student stress as a result of academic pressure is definitely real and can come with some severe consequences.
As a teacher, the most prominent "symptom" of academic stress that I observe is lack of sleep. Students often stayed up most of the night (or all night) to study for big tests and finish major projects. As result, most are not physically or mentally in any condition to learn and participate in class. Also as a direct result of sleep deprivation, most teachers could say they notice a high number of student sickness right around the time of final exams and final grades. Stress leads to a weakened immune system. Academic stress has also been linked to adolescent depression and relational tension. Like any other kind of stress (in teens or adults), if it is severe enough, and prolonged, it can lead to many other health problems.
If you are interested in scholarly studies, debates, and analysis on this topic, Internet resources are almost endless as it has been a topic of discussion and debate for many years. Most recently the discussions stem from the rise in competition for academic success (especially for college-bound students) as well as increasing political expectations of our nation's public schools. Though many other factors could be taken into account for a teenager's time management and stress level, homework amount and academic expectations are easy to track and control.