She began as a second-grade teacher, moved to teaching high school English, graduated to principal.
Meanwhile, nuns swapped their habits for ordinary clothes, became known as "women religious," and, in many cases, left their religious orders.
By McGrath's count, of the 63 women in the novitiate with her, only five are still sisters. When she arrived at St. John Fisher in 1986, there were 10 other nuns on staff. Today there are none.
"Should I be doing that too?" she asked herself as her friends left their religious lives to marry and have children.
She wondered, and she stayed, convinced that this was where she could matter, even though it "disappointed" her that the Catholic hierarchy often treated women as if they didn't matter.
"She's a very modern thinker," says her brother Dan McGrath, a former Tribune sports editor who is now president of Leo High School. "If women were to be ordained, she'd probably be first in line."
Sister Jean McGrath is 69. Ordination is probably not in her future.
And she thinks about the future as she watches her friends, the ex-nuns, have grandchildren.
"To whom do you belong?" she said.
Sitting in her office, the students gone, the hallways quiet, she contemplated the question, wondering what her life might have been like if she'd made the choice of the nuns who left. But she didn't. She had other things to do.
And so she belongs to the kids she has taught, the families she has counseled, the teachers she has guided.
As I was about to leave, she walked over to her desk and pulled out her calendar, a yellow book with a Mary Oliver poem paper-clipped to the front. She reads the poem every morning.
Now she read it aloud, repeating the final lines:
The gospel of light
is the crossroads of — indolence, or action.
Be ignited, or be gone.