Is it Atticus that teaches his children the most important lessons in life?

What an interesting question!  I'd agree with my
colleagues, of course, that Atticus--by words and example--teaches Jem and Scout most of
the lessons they learn in the course of this novel.  I'd also make the case, though,
that others were responsible for some "life lesson teaching," as
well.

Dolphus Raymond showed the kids (including Dill) it's
possible to live beyond the stereotypes.  He's still having to do a ridiculous thing
like let thewhite world think he's a drunkard, but through him they're able to see
beyond that to the reality that color is relatively unimportant to living a contented
life--and the unnfairness of having to hide in order to be
content.

Miss Maudie demonstrates, in word and deed, what
it means to be a true believer and follower of Christ in thae face of hypocrisy (in the
form of the "foot-washin' Baptists").  She also helps them discover that their view of
Atticus as a feeble old man who can't do what other "cool" dads do is
flawed. 

Calpurnia takes them to church and is able to show
Jem and Scout a world they really don't understand much about.  They had assumed all
other Negroes were educated (could read), but they discovered that's not true--yet they
are perfectly able to worship together through linin'.  They also encounter Lula, a
Negro woman who treats them with prejudice, so they're able to see this ugly attitude is
a choice, not something inherent or inborn or specific to any one
race.

While Atticus is their primary teacher, as he should
be, others do contribute to their learning experiences.