Since your co-workers are automatically the often-unwilling recipients of your classes if you call in sick, it's not a good idea to do so unless you really are sick . Hangovers, or just having a bad day, won't be acceptable excuses for missing teaching, to either coworkers or management.

Legally, for any sick day taken, your employer can deduct a day's salary unless you present an official note from a doctor. In practice, most employers don't ask for this note unless someone has been absent for more than one day or has a history of one-day illnesses.