I think we're forgetting the relevant reason why Sophia looks so lustfully at Zac, and he at her.
"Just yesterday," Aiden forced Sophia to remain semi-undressed in the middle of class, unbutton her tight blouse, and show all her tits and nipples in close-up to Zac, and if that weren't enough, he got Sophia to let himself go. groping and squeezing a breast firmly by Zac. And she felt very warm, although still "strange" because of the intensity of that situation.
That gives an important intimacy between him and her... and I think there is already a tension there (one more in our very bitch heroine of the saga, LOL)... that is easily resolved: Zac wants more. And Sofia also wants more from Zac.