Here some links to the concepts Mary has outlned. You did not say where you looked in the 1800 pages
Activity Locking - the contributed code modules (usually does not upgrade gracefully) that Conditional activities will sort of replace in Moodle 2.0.
Lesson dependency is a setting in the Lesson module. Grade is one dependency.
Not exactly what you are talking about, but in a similar manner, you can set up the navigation of pages in a Lesson module such that a student can not complete (or advance past) a "section" (or group of pages) until they answer some key questions correctly. This can be done by Jumps that are associated with choices (answers) students make. While each answer can be scored differently, the key with this concept is the jump, as opposed to a grade (say 70%) which guides/allows the student to go to a new area.
Best Chris