Do you capitalize PRINCIPAL?

You do not capitalize principal unless it is the first word of the sentence.
Example: Principal Smith taught English for two years.
Example: I introduced myself to principal Johnson on Monday.

 http://communications.med.nyu.edu/advertising-branding/editorial-style-guide/spelling-and-usage-common-problematic-words
http://www.umc.pitt.edu/styleguide/c.html

Sophie Daily
Writer, Three Penny Press