If you're a teenager who needs an excuse for drifting into dreamland during an early morning lecture, blame your circadian rhythms. Print out and keep this article from the Yorkshire Post as evidence.
One of the most fascinating sessions at the 'Creativity or Conformity?' conference I went to recently was by Russell Foster (Professor of Circadian Neuroscience at Oxford University) in which he explained how young people's body clocks run to a different rhythm than older people's. The result, when combined with 9am lectures and part-time work, is a lot of chronically sleep deprived young people.
Of course your lecturers have an equally good excuse for yawning through your late afternoon presentations!