Because they haven't figured out that Visine and coffee are a stoner-student's friends.

Because of a Week 8 conversation like:

Student:  How many pages is this paper supposed to be?

Instructor:  That's in your assignment sheet.

Student (takes out syllabus):  You mean this?

      
   (Yeah, week 8.)

Because before you've even started class, the mere thought of learning has so exhausted them, they must slouch down in their chairs and breathe through their mouths.