Jenny had been sacrificing victims according to a celtic symbol divided into five categories: warriors, virgins, healers, philosophers and guardians. She attempted to kill Lydia, leading to one of Lydia's patented screams. Only this one was heard by everyone in the school. It also revealed to Jenny just what she is.
“Unbelievable," Jenny marveled. "The wailing woman. A banshee, right before my eyes.” Hollywood Life's Andy Swift loved seeing things coming together on the show, calling the banshee reveal "so much cooler" than anything he could've predicted.
Jenny didn't think it so cool. Despite her realization, she was going to kill Lydia anyway. That is, until Stile's dad -- the good sheriff -- showed up and saved her. In doing so, he finally learned the truth about Scott and werewolves. Actually, he'd learned the truth earlier when Scott told him, but this was the first time he actually believed it. Seeing is believing, after all.