Sam watched the whole scene with a bittersweet smile on his face. He'd gotten his way, teaching them both a lesson. His revenge didn't completely clear the hurt from his heart, but it helped quite a bit.
As his anger faded, Sam was acutely aware of how aroused the whole situation had made him. His cock throbbed, growing engorged again as he stared at Jen's tight ass and firm breasts. His clothing began to feel tight and confining as the preliminary stages of the change began to happen. He shook his head from side to side, trying to clear the sudden fog that had come over his conscious thoughts.
It was horrifically tempting to reach down his pants and stroke his member, he found. Freedom from his human form was quite literally within his reach.
He pulled himself away from the gate, hoping a quick walk down the hallway would help him calm down. "Perhaps," he told himself,"if I simply get away from her, the mood will pass."
But as he walked down the hall, his member didn't quit throbbing, and all the walking did was simply to remind him that he'd feel much better on all fours. He felt a slight rustle as a few hairs began to sprout on his chest, and his cock began to burn with the animal desire for release. Sam fought hard against the change, paying a price in pain for fighting the beast inside himself, doubling over as his muscles cramped. He fought it down momentarily, as his cock softened briefly due to the pain. The hairs retracted back into his skin, and he breathed a small sigh of relief.
Much to his chagrin, however, the moment the pain subsided, he felt the all-too-familiar sensation of himself growing hard again. Sam knew the beast would not be denied, after having been so close to the surface so many times today. Sam sighed, resigning himself to the fact that there would be bloodshed at the school today, and if not in the school, somewhere in town.
As the changes began anew, he didn't try so hard to stop them, rather he simply tried to hold them off until he could find somewhere more appropriate to allow the beast out.