A doctor's excuse is required, regardless of the number of days of absence, for the
following communicable diseases:
measles, mumps, whooping cough, respiratory
streptococcal infections, scarlet fever, impetigo, conjunctivitis (pinkeye), ringworm of
the scalp, scabies, and infectious mononucleosis.
  A note from the doctor is also
required in order to be admitted to class if a student has been absent three (3) or
more consecutive days.  

A doctor's note should also be presented in order to be excused from gym due to
illness, injury or surgery.
When do I need a Doctor's Excuse?