This is
When I Was Twelve by JoMarch. And it was a hard one to find, let me tell you!
Here you go...
“Are you going to tell me where you’ve been, Clark?”
He thought only a minute before shaking his head. At this point, it wasn’t any of her business. “I’d rather not – ”
“Because you and Superman have both been gone for over six months.” She stared at him, her eyes piercing. “Almost to the day.”
He knew then. She knew. She already knew what he’d come all the way back to Metropolis to tell her.
