Hogan — affectionately known as "Father Phil" — is the last Franciscan priest at the school, where he's worked since 1964, two years after its founding.
"No one really knows how old he is," said athletic director Lawrence Nalls, a 1988 graduate who's served in various capacities at the Catholic school at 4930 S. Cottage Grove Ave. for 20 years.
"Good luck trying to find that out," said senior Andrew Williams, whose father and eight uncles all graduated from Hales. "But however old he is, I don't know what's in the water he's drinking, but I want some of it."
For the record, and after some serious prodding, Hogan revealed he's 77 — born on Christmas Eve, 1935.
Hogan said he even though he's "very old," being around teenagers on a daily basis has kept him young at heart.
"I keep saying to myself, 'When do I get a chance to grow old?'" said Hogan, who teaches a religion class to seniors, coaches the chess and bowling teams, and formerly taught music and instructed Hales' choir.
Saturday, he will conduct Mass for the school's graduation ceremony.
When Hogan, a Minneapolis native, arrived at Hales in 1964, it had 20 Franciscan priests who taught or performed maintenance work.
As the years passed, those priests — and others who followed — either transferred or died, leaving Hogan as the only one remaining.
"He's a walking piece of history," said Nalls, of South Shore.
Hogan even lived on the school's third floor for more than four decades before moving to a house with four other Franciscan priests in Hyde Park in 2009.
Much of his free time has been devoted to Hales students, like when he took a group to the Illinois State Capitol in 1969 or when chaperoning educational field trips to China, France and other overseas destinations.
"Hales has been his life," said Joe Moffa, a former Hales principal who is now on the school's board of trustees. "He hasn't known any other assignment."
Hogan said most Franciscans don't stay at their assigned locations for more than a few years. Hogan is the anomaly, a white priest who has spent nearly a half-century at a school with an almost-all-black student population that was founded to give minorities an opportunity to have a private-school education.
"I've stayed here because there wasn't anywhere else that had the same sense of appeal," Hogan said. "I must admit that I've been indispensable at times, and I worry what will happen when I'm not here.
"I'm one of the links to the past. Does it make any difference? I think it does."
Perhaps surprisingly, Hogan said he never had a "calling" to the priesthood and never thought he was meant to be a teacher.
But Williams, who's ranked second in his class and will attend Loyola University New Orleans in the fall, said Hogan is a natural instructor who "doesn't shove religion down our throats.
"He teaches us the history of religion, why it happened, but he's very open to other beliefs," said Williams, of Hyde Park.
Hogan has no plans to slow down. His mother lived until she was 93; an aunt recently died at 99.
And the word "retire" isn't in his vocabulary.
"My life has not been very dull, and I can't believe that other people wouldn't want to do what I do: to be a Franciscan, to be a Franciscan teacher," he said. "I guess it's just one of those lucky things that happens."