When Harold and Frances Gates went to their high school senior prom in 1960, he gave her an engagement ring.

She said yes.

After 51 years of marriage, work, children, grandchildren and great-grandchildren, the Houston couple retired and decided to take on a new challenge: They went back to college together.

In an interview with ABC News TV affiliate WTRK-TV in Houston, Harold Gates explained his motivation for returning to college at his age was "probably for the self-satisfaction that I'll have a degree."

The 69-year-olds are enrolled at Lone Star College in Kingwood, Texas, and they expect to get their associate's degrees next year.

The Gates are excellent students. They maintain a 4.0 grade point average, and proudly wear their Phi Theta Kappa pins. Phi Theta Kappa is the honors society that recognizes and encourages outstanding academic achievement by two-year college students.

"Oh, this was an honor when we received our invitations," Frances Gates said of being invited to join the group.

The college's counseling director, Kathy Adkins, told WTRK that the Gates are considered overachievers.

"Once they got into their classes, they found they were capable of doing this and far more, and I said, 'You know what? The sky's the limit,'" Adkins said. "In a sense, they hadn't given themselves enough credit."