I thought I could share some of my personal experience here.
I am a student in a French school of engineering (pardon my english mistakes).
This year, we had a very basic Java course (~30 hours).
The lessons were based on a 150 pages document, written by the school's teachers over the years, that had turned into something quite bloated and useless ("open the document page 36, then read page 17").
Teachers couldn't point out what students would find hard to understand about OO programming, so the document was very precise on the more advanced points, but very raw on the basic principles (what a Class is).
I am part of my school's studies council and wanted to contribute to the school's pedagogy.
So I had a meeting with the teachers and proposed to rewrite the document.
Glad that a student would be willing to play an active role in his education (and thinking that would be less work for them), they accepted.
And I started working. ...
The current version of the document is a 36 pages LaTeX file.
It took me +100 hours to write it ... and I am still not satisfied with some points and illustrations.
Point is : writing a course is WAY more work than I had thought.
And a good assignment is not easy to write. Changing it each year would be a huge PITA.