students have misspelled in their daily writing (personal journal, Writer's Workshop)
students have chosen to learn because they've asked me "how do I spell...?"
students have misspelled from a core word test.
Students have chosen to be their Big Time Challenge Word

  Students are required to add one challenging word a week. In order to add a new "Big Time Challenge Word" to their lists, students must know how to pronounce the word and know the meaning. Students should also have a reason for wanting to know how to spell it, if they don't I'll suggest choosing another word.