The perception that things are more difficult in high school than in the past is, of course, a subjective since the course work is clearly not as rigorous as in the past. One only needs to look at the reading level on which high school history texts are written compared to that of fifteen or twenty years ago. The geometry books of today are not as concerned with theorems as in yesteryear, either, and are simplified. For one thing, many schools do not require students to memorize formulas, theorems, etc.
However, science is certainly more complicated than in the past, and there are courses now that did not exist when parents were in high school such as those relating to technology, such as keyboarding, computer science classes, etc. More courses are available, and different courses are required, too, than in the past. In addition, students are involved in many other activities: sports, community service, clubs, and so on. Thus, while the course work is not as difficult, it is more extensive, and, as mentioned above, the stress-level for contemporary students is much higher than for those students of yesteryear.
With approaches toward learning geared to meeting standardized test score demands, the apprehension of concepts has taken a back seat to fragmented teaching of skills so that students will do well on state examines. As a consequence to this approach, students do not learn for the long term. Then, when students reach high school, they really do not have the foundation in learning that they need to succeed. When they have been allowed to use calculators and have not memorized multiplication tables, when they have not memorized formulas and theorems, students do not have a comprehension of how numbers work, so they cannot conceive how more advanced problems should be solved.
When they have read little and written even less and rarely discussed literature in their previous English classes because so much time has been spent drilling and conditioning them for the standardized tests in language, students are ill-equipped for the advanced skills needed in high school. This lack of preparation--as well as not having to learn how to learn through struggling on one's own--for high school may be the single most contributing factor for the perception by students of high school being "harder" nowadays.
Even for good students, though, there are new challenges as many schools are mired in old, traditional methods that worked well with older generations. The students now of the Digital Age think differently from their parents; consequently, approaches to teaching must be altered. Students are grouped more heterogeneously nowadays than heretofore so that "no child is left behind," and such grouping presents new challenges, also. In short, there are so many varied factors that enter into the equation that high school may seem more difficult to the present day-students.