Nearly doing a double take at seeing Lydia once they'd entered the classroom, Stiles had stopped and helped prop Derek onto the teacher's desk for leverage as he made to move and approach Lydia away from the rest of the group. "Lydia!" he whispered frantically, hands twitching nervously and ultimately hiding behind his back to be out of view - he was pretty sure there would be blood on them; oh god, Derek's blood. "What are you doing here?" he questioned, turning his body to check on the others as he did so.
Derek's movement made him falter, eyebrows pinching together when he saw Derek headed for the door. "Are you out of your mind?!" Was this guy actually for real? "Do you have a hard-on for death or something? You literally JUST healed and you're going back for round two?" He was aware of breaking the unwritten rule as he spoke, though since both Shauna and Trace had been present for the first somehow magical healing, Stiles didn't think it was too taboo - Lydia, on the other hand, might be confused.
Though Stiles didn't necessarily have any reason to be loyal to Derek, he was still incensed by his decision to go on this suicide mission - but also touched, in a way. Derek was willing to lay down his life to protect the group of them despite clearly not being an even match for this other creature seeking blood, and if nothing else, it was this that kept Stiles from arguing with Derek any further. As much as Stiles wanted to keep his dad as far away from whatever this was as possible, he knew he couldn't in good conscious just leave Derek here to die alone. "I'm going to call for backup," he said as he met Derek's gaze, hand reaching back into his pocket to first dial out to Scott while rounding up the group closer together. "Hey, don't die," he said in parting before Derek had the chance to leave the room.