The school girl who became an actress was Donna Reed. Her teacher, turned scientist, was Dr. Edward Tompkins.   The first indication Miss Reed had that her friend was associated with the atomic project was the newspaper headline revealing the bombing of Hiroshima, and the story telling of the important research done at Oak Ridge, Tennessee. A previous letter from Dr. Thompkins had been postmarked "Oak Ridge".   The actress promptly sat down and wrote a fan letter to the scientist. He responded with a letter telling of his work and then setting forth a startling suggestion. As a member of the Association of Oak Ridge Scientists, he was anxious to acquaint the people with the potentialities of the weapons they had helped create. This understanding was necessary for a quick and sure control of the bomb, they felt.   "News releases, magazine articles, pamphlets and even a book have been or are being prepared by our members," he wrote. "We have made a good start but much remains to be done. We are still largely failing to reach the 'man on the street.' "   With this in mind, he proposed a motion picture. "Do you think a movie could be planned and produced to impress, upon the public, the horrors of atomic warfare, the fact that other countries can produce atomic explosives and the vulnerability of civilization to attack by these explosives?"   Donna Reed read the letter. Then she showed it to her husband, Tony Owen, a former agent well versed in the ways of picture making. He saw in it the idea for not only an important. but also an outstanding motion picture. Fairly leaping to the telephone. he called his close friend, Producer Samuel Marx at Metro. Goldwyn .Mayer. The next morning, they sat down to breakfast together at the Beverly Wilshire Hotel, Dr. Tompkins' letter spread out before them.   Thus did a high school friendship in Iowa result in a motion picture being filmed that has been labeled by government officials. military leaders and scientists as the most important undertaking in Hollywood history.  "Good luck — I'll be six miles further away"