Looking for short story probably golden age about a father that enrolls his child in a school for psychic genius mutants. He is a latent mutant so he is able to see the secret writing the school uses. The school is part of an effort to fast track evolution in the human race because of fears of an alien invasion that is also evolving quickly.
This may be "The Children's Room" by Raymond F. Jones, although some of the details slightly diverge. I searched for it here. You can read it online here, or it's been collected in a number of anthologies. The basic premise of the story is that Bill Starbrook, an engineer, happens upon a book that his son, Walt, got from "The Children's Room" at the library. Walt comes down ill, and asks Bill to return the book. Bill finds out that there is no "Children's Room" at the their library according to the front desk, but he does find it on his own.
“Good morning, Miss Perkins. I wonder if you’d see that this gets to the Children’s Room for me? It’s due today and Walt’s sick.”
Miss Perkins smiled a good morning, then frowned. “The Children’s Room? We have no children’s department.”
Then, as he was almost at the door, he glanced to the left and swore softly. There, over a doorway, was the designation: Children’s Room.
After further perusal of the books, he happens upon the truth in one of them:
And then the answer came swiftly and like a sudden burst of flame before his eyes. He opened the second of the two volumes which he had not been able to comprehend before. Its words were plain now and addressed directly to the reader.
“You can easily comprehend, now, that you are a mutant.”
He stared at the words, trying to shed their meaning from his mind, but they stayed, and he knew the truth of them.
“You have come far enough to understand what that means,” the book went on. “You are aware of the extraterrestrial radiations which are continually producing mutations, and you understand some of the processes by which they are formed. It is not difficult, therefore, for you to understand that you are one of the many thousands of the ‘different’ men, the mutants who throng the Earth, scarcely knowing that they differ from their fellow in any matter.”
“In a time far distant from your own, the human race is in competition with another major race in the galaxy who are out-evolving mankind. In order to maintain not only the superiority which the human race has gained, but its very existence, it is necessary that the natural process of evolution be speeded. Wasteful and ghastly experiments have proved the impossibility of doing this by artificial means. Only through natural processes which cannot be duplicated at will can evolution proceed in an effective manner. But nature, in her waste of precious mutations throughout the ages, is herself responsible for man’s dire position in this future day.
“OUR purpose, then, is to accelerate the evolutionary rate of the human race by salvaging the beneficient mutations which have been wasted through the ages.
“You who have come this far with us have a duty now, a duty to join us, to bring your mutated characteristics before the race for the benefit of all.”
And, ultimately, his is an extremely unusual case, both that his mutation is present as an adult, and in that he carries no other mutation that is beneficial to humankind:
“Your case is most unusual. The total of your mutations consists only of the sensory characteristics by which you were able to read our mutation language, and find our way into the Children’s Room. I don’t recall a single instance previously where this mutation was not linked with some other. It is somewhat interesting from a purely biological viewpoint, particularly in view of the fact you are the father of Walt. Practically, however, your mutation has no value whatever.”
The children are ultimately brought forward into the future, and Walt and his wife receive a replacement Walt, a "homolog", indistinguishable from the original.