Yamamoto is lucky that the classroom door doesn't squeak. From the hallway it's hard enough to manage stealth with a group of three, even if Reborn is perfectly silent on his shoulder and Tsuna is hovering by the doorway instead of coming in. Even with a book to distract him, Gokudera is usually aware enough of his surroundings to notice someone approaching; if they get close enough to achieve their goal, it will be from luck as much as skill.
Luck, as it turns out, proves to be on Yamamoto's side. The sun is streaming through the window behind Gokudera, preventing the telltale of shadows from giving away the other's presence, and there's a tiny gust of wind from the pushed-open pane, the faint murmur of other students outside helping to disguise Yamamoto's approach. And Gokudera is truly entranced by the book on his lap, his attention written into the casual angle of his arm on the windowsill and the relaxed slump of his shoulders as much as demonstrated by the glasses he almost never wears in public and the tie holding his hair away from his face.
Yamamoto knows he should take the picture. It's a lovely angle, he can tell without any experience, is certain that he can capture that uncommon gentleness at Gokudera's features without any skill beyond holding the camera steady. But it's hard to look away and down at the camera in his hands, harder still to pull the lens up in focus, and Yamamoto ends up lingering instead, drawing the view up slow, across the easy angle of Gokudera's knees and up to the slouch of his shoulders. He lingers on Gokudera's unsuspecting face for a long moment as everything in his thoughts but appreciation goes blank, until his finger stalls on the capture button and his breathing catches short in his chest.
Yamamoto's not sure what it is that tips Gokudera off. He might make some unconscious sound, a tiny affectionate sigh escaping past his lips. Maybe Gokudera just feels the sustained stare, intuition picking up on the focused heat of Yamamoto's gaze. Yamamoto can see the tension appear in his features as he blinks, suspicion creasing his forehead as his eyes come up from the page to stare straight into the lens of the camera, and he knows it's too late but he pushes the button anyway, hears the shutter click just as Gokudera starts to shout, "Hey!" and move from his chair.
"Time to go," Reborn says in Yamamoto's ear, and the camera is sliding from his hold as Reborn tugs it free and hops away to make for the door. Yamamoto doesn't even turn aside to watch him go. Tsuna might be making a photo album, but even the best picture can't compete with the flushed-embarrassed reality in front of him. Gokudera's hold rumples his sweater, Gokudera's protests ring in his ears, but all Yamamoto can see is the shine of his eyes behind his glasses and the warm damp at his lips, and he doesn't bother offering any defense besides laughter and upraised hands.
He'd let Gokudera do anything to him, anyway. By now, he's pretty sure they both know that.