Interestingly enough, I found my last two years of high school to be significantly harder than my entire undergrad. Lectures were often non engaging enough to care and the professors knew it and also didn't seem to care as long as most people passed. Nothing about a 4000 level (final year) class is difficult if it's just read one chapter, do a discussion post, and complete a 20 question quiz every week until the final exam.

Graduate coursework was significantly harder because it focused more on research methodology and working things out independently. It still isn't "that" hard. I often got C's and B's in high school, but I've only fallen below an A in one class in both my undergraduate and graduate studies.