By Eric Hoover

Falls Church, Va.

Bob Nissen arrives at 6 a.m., long before the first-period bell sounds. On his belt, a ring holds the keys to 49 doors. Each morning brings something different—a leak in the ceiling, a cracked window.

Everywhere he goes these days, there are classrooms. As the maintenance supervisor for the Falls Church school system, just outside Washington, he oversees four buildings and their utility systems. At night he takes classes at George Mason University, where he is pursuing a bachelor's degree in educational psychology. One day that degree will lead him to the classroom he has yet to see: the one where he's the teacher.

Mr. Nissen, 52, is among a growing number of baby boomers who can call themselves college students. Some have enrolled after losing their jobs. Others have sought to escape the boredom of retirement.

Mr. Nissen had his own reasons. Since coming here, in 1987, he has worked around teachers each day, observing how they interact with students. He saw how residents of this small, suburban community admired those who taught their children. "I always felt like I was part of education," he says, "but not directly involved in it."

After graduating from high school, in 1974, Mr. Nissen worked in construction. Then he went to trade school so he could work in the heating and air-conditioning industry, which led him to his current job. Most of the time, it was satisfying work; things needed fixing and got fixed. He felt good when he went home.

Still, that feeling was different than the one he would get while working at one of the school system's two elementary schools. Some mornings, he would stop to watch students filing off the bus. "There's all this bright energy around them," he says. "They are the cleanest sheet of paper you could ever have."

The idea to become an elementary-school teacher came slowly. Over the years, as he attended retirement parties and watched his two children grow up, a feeling seeped into him. Call it restlessness. By 2003, when his son, Robert, left for Christopher Newport University, he knew that he, too, wanted to go to college.

The following year, Mr. Nissen enrolled at Northern Virginia Community College, the first stop on a long road to the front of the class. His first two courses were biology and an introduction to speech communication. He looked around for students his age but did not see many.

In high school he had been an indifferent student, and this time around he wanted to squeeze something from each lesson. So he made sure to sit in the front row. In each new class, he memorized his classmates' names, practice for teaching.

At first studying was slow going. At his dining-room table, he would read a passage, write down notes, and then read it again. Only then did the information stick. His homework and papers took about six hours a week, sometimes as many as 12. There was less time for housework and side jobs installing air-conditioners.

An information-technology course threw him. All the younger students already knew Excel and PowerPoint. As he pressed on, he imagined the chemistry of his brain changing, bit by bit. "It was if it had been stuck in neutral for a long time," he says. "You have to work to get out of neutral."

Straddling 2 Careers

Mr. Nissen's courses have taken him all over the place. In art history, he learned how Titian and other Venetian painters captured the afternoon sunlight on canvas. In biology, he learned about DNA and how one gene regulates the way humans burn fat. In American history, he learned all the names and dates he had long before forgotten.

So far Mr. Nissen has earned nothing but A's. His wife, Susan, has teased him by threatening to throw him a "B party" if he ever slips up. She encourages him, as do his friends, though some of them still do not understand why he wants to spend his days in a classroom with two dozen little kids.

Mr. Nissen, who earned an associate degree last year, needs about 45 more credits for his bachelor's degree. That probably means three more years of classes. After that he must pass a state exam and find a job.

Until then Mr. Nissen plans to keep working. On a recent summer day, there is much to do at George Mason High School. This morning he cut his finger while using a table saw to trim a cedar board for a repair job.

Around 10 a.m., Gabriel Fernandez, one of the five employees Mr. Nissen oversees, stops to show him a small black box. It's a fan relay, which has apparently failed. Cold air in another part of the building will not stop blowing, Mr. Fernandez explains. Mr. Nissen inspects the device and tells him that he will look into it.

A while later, a peek into the basement reveals that one of the school's monstrous boilers—a 1951 Cyclotherm—is leaking. "This might need new valves," he says, moving a bucket over to catch the drips.

In Mr. Nissen's office, several e-mail messages await. Someone has requested big fans and extensions cords for an after-hours event. This afternoon he must complete a purchase order for a new trailer that will serve as a classroom. Outside his door, a brand-new condenser coil stands in a giant wooden crate; it's a replacement part for the air-conditioning unit up on the roof of the auditorium. He figures it will take two days to install.

His desk reveals the two careers he straddles. How to Talk So Kids Can Learn sits beside a catalog from Aireco, which sells heating and air-conditioning supplies, and a copy of School Planning & Management magazine.

Mr. Nissen always tries to leave by 3:30 p.m. On nights when he has classes, that's enough time to drive home and eat dinner beforehand. This fall he plans to take two courses back-to-back on Thursdays, which means he will finish at 10 p.m. He has considered taking a third class, but he's not sure he can manage the load.

So far his experience as a midlife student has left him with only one regret: that he did not start taking classes sooner. He might be 56 when he finally introduces himself for the first time as "Mr. Nissen" in his own classroom, most likely full of first-graders. Already he has decided how many years he wants to teach—12, long enough to see his first class graduate.

First in an occasional series on adults who seek higher education after 50.