Are the classes that top-tier school such as Harvard and MIT "harder" than classes are lower tiers?

[1NJParent]

…the strengths of peers at X Univ can allow a higher depth and breadth of teaching and expectations (tests, projects) than at Z Uni. The bar for an A grade can be higher, based on this. If peers come in with the highest preparation, you may not be comfortable with the pace and rigor they’re ready for. It depends.

I agree. Course rigor is generally determined by the strength of its students, especially the dispersion of strength/ability within its student body. For example, a school like Caltech can offer the most rigorous courses in math/science because of its student body’s strength and its low (probably the lowest) dispersion of that strength.