ive seen some tests from top tier schools and they were harder than my own and my school is no slouch. you can say what you want about grading but when you have top students, professors are going to push them harder and faster because they can handle it
When everyone in the class was valedictorian of HS or in the top 5% and they all expect to do extremely well, classes get extremely competitive and tests inevitably get harder. Professors do push these kids extremely hard because they want to break whatever mental boundaries students feel like they have. High stress environments produce beautiful work, in my opinion. Also I think professors love to humble students, something a lot of these top students need.
My point of view: kids who got into top tier institutions have been putting in an enormous amount of effort or have demonstrated high performance for a longer period of time (excelling in high school). It's a display of endurance. I think while gpa tends to sink at a few of these top tier institutions and it may not be ideal, there is zero point to getting higher education if it's not challenging. Obviously challenging the top 5% of high school students will require more difficult material. That's not to say people at state schools or wherever can't also grasp this material...they definitely might ( I don't know I only took 1 class at a state school back in soph. year of high school so I don't have too much experience), which is why top medical schools don't just recruit students from top universities. It's also something of a comeback chance in my view. So you didn't kill high school for whatever reason? Well try hard in college and you still have a strong chance at getting into a decent medical school. (there is also the possibility of top students attending state schools for family or financial reasons to consider...)
My problem with places like Harvard: with so much grade inflation and systemic cheating, it's definitely a shame that such great minds are put to waste. It's important to humble students and push them extremely hard. I don't like the idea of "just try very hard in high school, get into harvard, and life will be a breeze." I know this isn't the same for all Harvard classes, but all my crimson peers have told me the lowest grade in the class has been a B and prof's would never jeopardize their gpa's and futures.
So my argument doesn't necessarily clearly pick one side of the central question, but I would lean towards saying that
some top institutions grade much harder and adhere more closely to the philosophy that getting top students is an opportunity to see what the mind can do when challenged and put in high stress situations. I think it's a beautiful (but sometimes slightly unhealthy) phenomenon. And hey, that's kind of medical school, right? Get knocked down a few pegs every day?
I know I make it sound like top students are lab mice but I don't intend to imply that.
At the end of the day, this type of discussion can't really find a definitive answer to the question because there is no way to compare classes. There is so much variation from school to school and within institutions themselves. This is perhaps why adcoms consider the reputed rigor of the school and the classes at the school while making decisions (if they don't this is my not so subtle suggestion that they do so).
I told myself I wouldn't get further into this discussion but I did because the water for my tea is taking forever to boil. I need caffeine now!!