The Collaborative International Dictionary of English v.0.48:

Schoolmistress \School"mis`tress\, n.
   A woman who governs and teaches a school; a female
   school-teacher.
   [1913 Webster]

WordNet (r) 3.0 (2006):

schoolmistress
    n 1: a woman schoolteacher (especially one regarded as strict)
         [syn: schoolmarm, schoolma'am, schoolmistress,
         mistress]