> the professors started to introduce one problem in each exam that anyone could pass who practised a bit and / or wasn't entirely stupid.

What is considered a passing grade at one of these schools?

At my private liberal arts college in the US it was usually (but ultimately up to professor discretion):

  00-59% - Fail (0.0 for GPA)
  60-67% - D    (1.0)
  68-69% - D+   (1.3)
  70-72% - C-   (1.7)
  73-77% - C    (2.0)
  78-79% - C+   (2.3)
  80-82% - B-   (2.7)
  83-87% - B    (3.0)
  88-89% - B+   (3.3)
  90-92% - A-   (3.7)
  93+    - A    (4.0)

You could count any class with a D or better toward graduation, but to actually graduate you needed a 2.0 or better GPA. If you did not have it you could take additional courses to bring your GPA up, but adding on 3-credit courses when you've got 120 credits built up with a sub-2.0 GPA is typically a losing proposition. Most transferred if they were sub-2.0 by the end of their Sophomore year.

I've seen other grading scales with E's in addition to/instead of F, or minor variations on the percentages. It seems popular to give Honors courses an extra point (e.g. an A- is 4.7 instead of 3.7), particularly in US High Schools.