"Low Grade Ore", a novelette by Kevin O'Donnell, Jr., first published in Isaac Asimov's Science Fiction Magazine, Summer 1977, available at the Internet Archive.
Humans had been conquered or lost a war against aliens.
On the afternoon of 23 June 1979, five hours after the then-President of the US surrendered to end the Two Minute War, the Pukcip had staged a demonstration on the Kansas plain. Their small expeditionary force had slammed into Wichita with more fury than any tornado had ever unleashed; their grim sweep missed but four of the town's three hundred thousand residents. The rest lay rotting in the corn-growers' sun.
When it was over, their Commander had preempted the nation's communication networks. As cameras inched over the dark stains on the carapace he'd refused to cleanse, he said through his interpreter: "You see now our seriousness. Do as you are told, and all will be well. Oppose us even lightly, and a larger city will suffer the same fate."
The aliens demanded that human children be tested for psychic powers, or the earth government needed to find such children? The child was shown a very peculiar hallway with (I think) a tiled floor.
The far doorway called him. Scowling, he crossed to it, and scanned the holographic projection of the—reception room? laboratory? zoo?—on Pukci. All shimmers of swirling blues and purples, it stood in haughty contrast to the peeling green paint of the Center. Olsen glared at the metallic glints in the odd-shaped tiles of its floor, as if he could dissolve the deception through sheer force of will. The vivid image remained unaltered. The Pukcip equipment was too good.
Spurred by an impulse as inexplicable as fate, he stepped forward. The lieutenant's surprised gasp plucked at his shoulder, but he didn't respond. Gravity grasped him, ripped him through the planes of colored light. An instant later, he was bouncing softly on the nylon mesh strung behind the doorway.
They were sent out onto the floor. Turns out it's an illusion, and the child would fall through and land safely.
Two hundred fifty children ran, skipped, hopped, or walked into the Pukcip projection every day. Passage over the threshold triggered the test which, for one uncaring nanosecond, pitted the law of gravity against the child's belief that he was in a room whose floor could support his weight. If reality won, the testee tumbled. If faith overrode it to write its own version of natural law, the child either levitated in blissful ignorance . . . or teleported directly to the original room, somewhere on Pukci.
Roughly eleven of every million testees had faith enough to warp reality.
If the child had a very specific mental ability, then they would somehow create a space warp between the test location and the distant alien planet where the actual hallway existed.
They should have guessed. When the test provoked two different paranormal reactions, one less common than the other, they should have guessed that it might provoke a third, even more rare. This boy, neither levitator nor teleport, was something else entirely: a talent capable of wedding Earth to Pukcip via another dimension, and of keeping them joined, perhaps indefinitely.
Highly trained soldiers were on call in the test facility in case such a warp was formed, and they could move through and form an invasion beachhead on the alien planet.
Jonathan was standing on glistening tiles, apparently inspecting the spacious room. Doors opened in the shiny walls. Olsen saw Pukcip heads come around the edges. The boy sat down and started to sob.
Dortkowski gave a sound, almost a whimper, of relief, and sagged against him, murmuring, "It's a levita—"
But Olsen shouted, "Hell with that, it's a space warp! Get in there, you bastards, get in there and take that place!!!"
Within seconds, his and other booted feet were skidding across Pukcip stone; back in Hartford, his Spec 5 was demanding reinforcements for the bridgehead; and Olsen, automatic in hand, was cradling the terrified boy.