It's the power of directional anger. The teacher was the one to bring him up there. But having laughed before, his mind thought 'someone else will laugh at you now'.

Xion was just unfortunate enough to have been moving in that moment. A target that his mind decided was going to be next to laugh. (even if she wasn't)

He is definitely closer to the Jerk end of the general world scale, just by virtue of honestly having no idea how friends even work, sadly.

"...Xion," he repeats calmly, mentally logging it away. "Hmm. I, am Noa then," the boy introduces. "Kaiba, Noa. ...Planning to sit out here until the end of the class?" he adds, unable to keep from making extra remarks.