Letter Ettiquette Question

[lifesaver]

Quick Question All...

I have been asked to write a letter in support of a high school teacher I had some years ago. She has been nominated for a national award and she neeeds letters of support from former students, friends, co-workers, supervisors, etc. I will be writing one of many letters from former students. Being from a small town, in four years, I had this teacher seven times for different classes; (three english classes and four debate classes). She lives in a different town, but we have remained friends.

The letter will talk about what a great teacher she was and how we maintained friends for over a decade now.

My question is this; how do I refer to her in the letter? Do I write it from the perspective of a former student and refer to her formally throughout as "Mrs. Williams"? Or can I use standard letter format of peers (because we're friends now) and initially refer to her as Mrs. Williams and alternate between that and 'Barbara'? I now refer to her in real life and emails as Barbara.

Any help would be appreciated.