I subbed today for a high school American History/English teacher. I guess since those are (more or less) my certification areas I wasn't actually pretending today.
While I've never actually had to teach grammar, in any capacity, it turns out that I do know how find the singular (and plural) possessive form of words. Go figure.
Being able to tell a class, "all four worksheets, plus an optional fifth worksheet can be due by the end of the hour, at MY discretion" is a wonderful thing.
A less wonderful thing is when a student asks, "what does discretion mean?"