Please, only tight-laced editorial types respond (as if):

What is the hard and fast--if often broken and ignored--rule about titles in the address of a business letter, specifically the title of 'principal'?

Let me show an address in a business letter:

Mr. John Doe, principal
Wavering Heights High School
Rack-Driven Slip, VA 23838

Dear Mr. Doe:

...and so on.

OK. What is the most tightly-laced rule you know about the title of principal there? Is the hard/fast rule that principal should or should NOT be capitalized. I think most people nowadays always capitalize such titles, but I have this little itch of an instinct that tells me I wasn't taught to capitalize a title in such a case, oh, way back in about 1967. I sure would like to know the rule.

Many thanks,
WW