The way I look at it is:
Miss is for young girls or would be used if addressing as Miss Firstname - I see this used for preschool teachers/dance teachers/girl scouts, etc....
Ms. is for either married or single women & would be used as
Ms. HusbandsLastname or Ms. MaidenName. I see this used for elementary & higher teachers who are not married. This would also be used if you didn't know what Jane Doe's marital status is.
Mrs. is for married women they would be addressed as
Mrs. HusbandsLastname
From Wikipedia:
Ms or
Ms. (US usage) (pronounced /məz/ or /mɪz/) is a
title used with the last name or full name of a
woman. Unlike the more traditional titles
Miss and
Mrs, it does not bear any reference to the woman's marital status, as
Mr does not for a man.
Although it is usually believed to be a creation of modern
feminism, Ms was sporadically used as an abbreviation for the title "
Mistress" (just like
Mrs) as early as the 1700s, and the pronunciation
mizz for
Mrs was colloquial in the American South and other areas. Indeed "Mistress" originally did not bear reference to marital status either, until the title separated into the diminutive "Miss" and abbreviation "Mrs" in the
17th Century.
The use of
Ms as a title was conceived by Sheila Michaels in 1961[
citation needed], upon seeing what might be a typographical error on a copy of
News & Letters. Address-o-graph plates were difficult to repair and small, poor groups would not waste resources to correct minor mistakes. Michaels' roommate, Mary Hamilton (Congress of Racial Equality's first female Field Secretary in the South) had spoken to the Marxist-Humanist group in Detroit and taken their newspaper. Michaels, who was illegitimate, and not adopted by her stepfather, had long grappled with finding a title which reflected her situation: not being owned by a father and not wishing to be owned by a husband. She knew the separation of
Miss and
Mrs had been recent, but one could not suggest that women call themselves
Mistress with its louche connotations. Her efforts to promote use of a new honorific were ignored in the Civil Rights era, and seven years later in the nascent Women's Movement. Around 1971, in a lull during a WBAI-radio interview with
The Feminists group, Michaels suggested the use of the title
Ms (having chosen a pronunciation current for both in Missouri, her home). A friend of Gloria Steinem's heard the interview and suggested it as a title for her new magazine. The tape was erased for re-use by volunteers who regarded the Women's Movement as a joke.
The usage of
Ms was championed as
non-sexist language beginning in the 1970s, especially in business usage, by those who argue that a woman's marital status is of no relevance in such a context. Starting in the 1970s, many women chose to be called
Ms for political reasons, and a major feminist magazine is named
Ms.
The Times (
UK) states in its style guide that "Ms is nowadays fully acceptable when a woman wants to be called thus, or when it is not known for certain if she is Mrs or Miss."
The Guardian (
UK) states in its style guide that: "We use whichever the woman in question prefers: with most women in public life (Ms
Booth, Mrs
May, Miss
Widdecombe) that preference is well known; if you don't know, try to find out; if that proves impossible, use Ms."
Most women style themselves either "Miss" or "Mrs". However, in some circles the title is now standard, for instance in business and where one may not know or find relevant the marital status of the woman so addressed. The default use of
Ms is championed by a number of
etiquette writers, including
Judith Martin (
Miss Manners).
The title
Miss is now considered by some to be quite old-fashioned except for actresses, entertainers, most umarried women and young girls. The title
Mrs is still in common use, especially by women who have taken their husband's family name - the vast majority.
However English school children address female teachers as
Miss regardless of marital status.
Several public opponents of "non-sexist language", such as
William Safire, were convinced that
Ms had earned a place in English by the case of
Geraldine A. Ferraro. Ms. Ferraro, a
United States vice presidential candidate in
1984, was a married woman who went by her birth surname rather than her husband's (Zaccaro). Safire pointed out that it would be equally incorrect to call her "Miss Ferraro" or "Mrs Ferraro" or to confuse the reader by calling her "Mrs. Zaccaro"!
The rare plural of
Ms is
Mses It is not standard to use
Ms as a term of direct address; usually
Ma'am will be used.
In other
European languages, non-sexist usage in this regard usually amounts to using words more or less equivalent to
Mrs. (
madame,
Fr.;
señora,
Es.;
senhora,
Pt.;
signora,
It.;
Frau,
De.;
bean-uasal,
Ga. and
Gd.) for both married and unmarried women, and whether they take their husband's name or not. This makes sense as these titles are usually the direct feminine equivalents of the male titles (
monsieur, Fr.;
señor, Es.;
senhor, Pt.;
signore, It.;
Herr, De.;
máistir/
tiarna, Ga.;
maighstir/
tighearna, Gd.), whereas the equivalent of
Miss is a diminutive of the female equivalent (
mademoiselle, Fr.;
señorita, Es.;
senhorita, Pt.;
signorina, It.;
Fräulein, De.;
ógbhean-uasal, Ga.;
maighdeann-uasal, Gd.).
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
From The American Heritage® Book of English Usage.A Practical and Authoritative Guide to Contemporary English. [size=-1]1996.[/size]
Many of us think of
Ms. or
Ms as a very recent invention of the womens movement, but in fact the term was first suggested as a convenience to writers of business letters by such publications as the
Bulletin of the American Business Writing Association (1951) and
The Simplified Letter, issued by the National Office Management Association (1952). Along with many others, champions of womens rights saw the virtues of the term and soon advocated its use in more general contexts, as is evidenced by the founding of
Ms. magazine in 1972.[size=-2]
1[/size] The form
Ms. or
Ms is now widely used in both professional and social contexts. Thus the term stands as a highly successful language reformprobably because people value its usefulness. As a courtesy title,
Ms. serves exactly the same function as
Mr. does for men, and like
Mr. it may be used with a last name alone or with a full name:
Ms. Pemberton; Ms. Miriam E. Pemberton.[size=-2]
2[/size] Using
Ms. obviates the need for the guesswork involved in figuring out whether to address someone as
Mrs. or
Miss: you cant go wrong with
Ms. Whether the woman you are addressing is married or unmarried, has changed her name or not,
Ms. is always correct. And the beauty of
Ms. is that this information becomes irrelevant, as it should beand as it has always been for men.[size=-2]
3[/size] Of course, some women may indicate that they prefer to use the title
Miss or
Mrs., and in these cases it only makes sense to follow their wishes.
Hope the above helps!
I do remember though back in the late 70's, when I was in the third grade, that MS. Edwards made a HUGE production out of this. We got a lecture on the first day of class that we were NEVER to refer to her as Miss. That she was divorced and Edwards was not her maiden name, therefore we were to refer to her as Ms. Edwards. She was mean. 
