Salam,
I think it also depends on what types of classes you took in high school and how dedicated you were during that time. I recently talked to a friend of mine who attends AIMC, and he told me when it comes time to study for a test, it's pretty much like buckling down and studying for AP tests. Now, if you're someone who never took AP classes and got through high school taking easy classes and getting good grades that way, the odds are going to med. school in Pakistan will be really difficult because you do actually need to study and work hard to get good marks. On the other hand, if you took rigorous courses (like AP, IB, etc.), then chances are it won't be as bad as not having taken such courses. I'm not saying for those who took advanced courses that it will be a walk in the park. In fact, some people go through AP courses without stressing much and just barely passing -- they don't learn the info, but they just rely on review books to "pass the test" without trying hard all year. For such people it's going to be hard to adjust to because from what I understand, it's much harder to slide by like that without actually learning the material.
My point is, the best way to know how hard it will be is to assess what kind of a student you were in high school. If you felt that you truly worked really hard and had a good study ethic, you shouldn't worry too much. If you're accustomed to finding the shortcut to get good grades without studying much (because it is EXTREMELY easy to do that in U.S. high schools), you want to seriously consider whether med. school in Pakistan (or any med. school for that matter) is the right thing for you.