Oh please god just kill him right then and there. He didn't see the exchange of cum between Derek and Lily because Stiles was feeling to guilty to even look in Derek's direction.
Stiles did up his pants as soon as Lily was off of his lap and he was grabbing his stuff before Derek was even talking. "I will... I am going back to my common room." Ashamed, guilty, and embarrassed Stiles couldn't look at either Lily or Derek as he held his stuff to his chest, crumpling it, and moved to get away from them both.