Minerva McGonagall's background

[Salvador, Anjali]

No: HPFGUIDX 102770

"How long have you been teaching at Hogwarts?" Professor Umbridge
asked.
"Thirty-nine years this December," said Professor McGonagall
brusquely, snapping her bag shut.
-Harry Potter and the Order of the Phoenix, page 321 (American
edition)
 
Was anyone other than myself confused by this? Why would McGonagall
have started teaching in December? After all, she teaches
Transfiguration, and in CoS it says that Dumbledore taught
Transfiguration himself, so she, presumably was his successor. What,
though, would have made Dumbledore stop teaching in the middle of the
year? The Harry Potter Lexicon says that Dumbledore became headmaster
in 1970 or 1971-after McGonagall began teaching, because she would
have started in 1956. Unless, of course, she began teaching a subject
other than Transfiguration. This is possible, of course, but I find
it unlikely considering the fact that Transfiguration is undoubtedly
her specialty (evidence being that she is an Animagus).
 
This brings me to my next point-what did McGonagall do before
teaching? I find it incredibly interesting that she was only a year
ahead of Tom Riddle. The Chamber of Secrets would have been opened
during the last half of her sixth year at Hogwarts, not to mention
that Grindlewald would be coming to power sometime around this
period. Also, when did McGonagall become an Animagus? It makes sense
that Dumbledore would have taught her if she had learned at Hogwarts,
but does this mean that he is an Animagus as well? In addition, what
did she do after graduating-an Auror, perhaps? McGonagall was quite
vehement in her support of Harry's ambition to become an Auror; this
might have stemmed from being an Auror herself. She certainly must
have been a formidable witch; recall Pomfrey's "As if one of them
could have stunned Minerva McGonagall face on by daylight!" Did
McGonagall help Dumbledore in his defeat of Grindlewald? She would
have graduated by then and had two years to go through any sort of
training. They certainly would have known each other, given
McGonagall's gift for Transfiguration. I won't bring up the question
of why Dumbledore, a schoolteacher, would have been involved in
bringing down a Dark wizard in the first place.
 
This is my first post, so I apologize if these questions have been
covered in previous discussions.
 
Anjali