No one saw the superhero in mild-mannered Clark Kent.

Jim O'Connor keeps his students fooled too.

In his algebra and calculus classes at St. Francis High School, he is stern — no excuses, no coddling. "If you look at the clock," said senior Michael Tinglof, who had O'Connor in his freshman year, "you're on his bad list for the rest of the class."

The 70-year-old teacher's look also is all business: spine straight, close-cropped silver hair. When he cracks a joke, he's so deadpan that the boys often miss it, senior Pat McGoldrick said.

"Like in our class, he'll put a problem up on the board and then someone will say, 'Oh, can you do it this way?' And then he'll respond, 'Oh yeah, I'll just do this and I'll just change that and I'll do all this extra work and I'll get the same answer. It's totally worth it.' "

Until they get accustomed, Pat said, "everybody thinks he's being really mean."

For the record, O'Connor embraces the reputation. "You want to teach a class with 30 boys, you've got to be strict," he said.

Michael and Pat might never have found out how little they really knew about their teacher if they hadn't signed on this year to recruit donors for a school blood drive.

One afternoon, the boys took a field trip to see where the donated blood would go. In the hallways of Children's Hospital Los Angeles, they were greeted like VIPs because they were associated with one.

"He was like a celebrity there. Everybody knew his name," Pat said of O'Connor.

They discovered one reason when they went to the hospital's Blood Donor Center, which has a plaque ranking the top donors. O'Connor's name is engraved in the top spot, 50 gallons — though that total is way out of date.

Since he first gave blood at Children's Hospital in 1989, at the urging of a friend's wife who was a nurse there, O'Connor has donated more than 72 gallons of blood and platelets.

That enormous gift — worth well over half a million dollars had it been purchased — has been especially valuable because he is a universal donor. His O-negative blood can be given to people of all blood types. It can be used for newborns and, in an emergency, before a victim's blood is typed.

Once a month without fail, O'Connor arrives at the hospital's donor center to give platelets, which are vital for cancer patients undergoing chemotherapy, and for those who have had open-heart surgery or bone-marrow or organ transplants. It can take about two hours for a machine to draw his blood, separate out the platelets by centrifuge and then return the remaining components to him.

O'Connor also gives blood every other month, which is as often as regulations allow. He's been the hospital's top donor for years — by a long shot.

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O'Connor grew up in New York. He served in the Navy during the Vietnam War, doing electrical work on an aircraft carrier. Before becoming a teacher, he worked deep in the Holland and Lincoln tunnels as an electrician for New York's Port Authority.

College came late for him — and took a while to complete. He started with night school, graduated at 30 and came to California in 1973 to be an engineer at Hughes Aircraft. He coached youth sports on the side and enjoyed it so much that he decided teaching was what he should do with his life.

He spent a decade at St. Francis in La Cañada Flintridge, starting in 1976, before a 20-year stint at Harvard-Westlake. Rather than retire, he arranged to return to St. Francis part time. His schedule alternates from Monday, Wednesday and Friday to Tuesday and Thursday.

When he's not at school, he's usually at the hospital.