I think that you're getting at a great point, but another contributing factor may be that high school has a different scheduling structure.

  High School:
    Classes/Week   = 5
    Time/Class     = 45 min
    HW Frequency   = Every Day or Every few Days
    Exam Frequency = Every Week or Every Couple Weeks

  University:
    Classes/Week   = 2 or 3 + Discussion
    Time/Class     = 90 or 120 min
    HW Frequency   = Every Week or Every Couple Weeks
    Exam Frequency = Every Month or 3 per Semester

All I am trying to say is that difficulty of material is not always the problem. Sometimes the system can be the problem. It's clear that university expects you to spend more time doing independent study given there is more time between sessions, yet many new students aren't accustomed to spending their free time this way.