Nancy Reagan yesterday returned to her childhood alma materthe Chicago Latin School, a coeducational private institution (it was named the Girls Latin School when she graduated in 1939). Mrs. Reagan started off her trip with a visit to Pat Peterson's third-grade class, which, as part of a lesson in letter-writing, had invited the first lady to visit.

"I can't tell you how wonderful it has been for me to come back here," said Mrs. Reagan as she accepted a set of matching school sweat shirts and caps for herself and President Reagan. "When you get to be as old as I am , then you will understand what a special feeling it is to come back where there are so many memories."

The first lady's visit was not without some rather grown-up questions from the schoolchildren. George Perry, 9, asked the first lady what impact the economy is having on people. "That would get us into a real long discussion," Mrs. Reagan answered. "You cannot undo in one year what was done in four years."

"Nobody told me to ask it," George said later. "I want to be a doctor when I grow up and I want to know how much they're paid."

After lunch in the school cafeteria, Mrs. Reagan went on to Northwestern University Medical School, where she established the "Loyal and Edith Davis Professorship of Surgery" in honor of her parents.

Roy Cohn, once chief counsel on Sen. Joe McCarthy's subcommittee to root out communist infiltrators, is back at work. The controversial attorney has joined the advisory board of Western Goals Inc., an anticommunist educational group established in 1979 by Rep. Larry McDonald (D-Ga.) to monitor incidents of alleged terrorism and report on the spread of communism.

McDonald aide Tommy Toles said that Cohn and McDonald have been friends "for a number of years" and that McDonald was not concerned about Cohn's past association with McCarthy.

Author J.D. Salinger, rarely seen outside the seclusion of his Vermont home, made a surprise appearance to visit actress Elaine Joyce at a dinner theater production in Jacksonville, Fla., of "6 Rms Riv Vu."

Salinger went to the Alhambra Dinner Theater to see Joyce after a three-month exchange of long-distance phone calls and letters. He stood on the sidelines after the show Tuesday night and watched her sign autographs and pose for photographers.

The 63-year-old author, whose novels include "Catcher in the Rye" and "Franny and Zooey," refused to pose himself.