Wrzesniewski, A., McCauley, C., Rozin, P., & Schwartz, B. (1997). Jobs, careers, and callings: People's relations to their work. Journal of Research in Personality, 31, 21-33.

Abstract (from article):

We present evidence suggesting that most people see their work as either a Job (focus on financial rewards and necessity rather than pleasure or fulfillment; not a major positive part of life), a Career (focus on advancement), or a Calling (focus on enjoyment of fulfilling, socially useful work). Employees at two work sites (n = 196) with a wide range of occupations from clerical to professional were unambiguous in seeing their work primarily in terms of a Job, Career, or Calling. Differences in respondents’ relations to their work could not be reduced to demographic or occupational differences; an homogenous subset of 24 college administrative assistants were, like the total sample of respondents, distributed evenly across Job, Career, and Calling.

It's easy to imagine that people might see their work as a job or a career, but it's interesting that any occupation can be seen as a calling although some occupations may more readily seem to fit the role of a calling.

Doing a quick Google search for "teaching calling" pulls up quite a few sites that consider teaching a calling, a career, or a job.

For myself, it's difficult to see any occupation as a calling. People's background and experience make them better at some occupations and worse at others.

At the same time, there seems to be a need for schools to have a particular attitude, a culture of caring and respect for their students (see also What Works in Teaching). Such a culture should result in better job satisfaction for the teachers (and perhaps prevent the high drop out rate of new teachers) and better success for the students. And it seems likely that how one perceives teaching—job, career, or calling—might have an effect on a school's culture.