This facial expression is very often associated with the reaction of the nipples to various stimuli.
Oh yeah but no wonder Sophia bites her lips so much, she hasn't had a good fuck in over a year, too much juice in the system is bad for her health, at least Amber will get them out of hers in the next update
Well, actually while doing the compilation I noticed that the lip biting routine is not about the nipples as often as I also thought.
Here is the breakdown by PD, same order as the images that I posted.
PD 4
* Sophia spying on Nathalie while changing 1st day of work - Aroused by the Nathalie changing bare breast
* Sophia solo night - Sophia finger brushed her clit watching the porn movie
PD 7
* Sophia spying on Liam and Amber having sex - Aroused by Amber's boobs bounce up and down
PD 10
* Sophia spying on Nathalie jilling off on Sophia's bed
* Chicken fight (Amber)
* Fashion show with Dylan and Sam 1 - she notices that they are watching her crotch
* Fashion show with Dylan and Sam (and with Dylan alone too) 2 - combination of being exposed, the fabric and the inevitable breeze get the nipples hard
* Shower with Ellie - Ellie groping her butt
PD 11
* By the pool with Sam 1 - Sam massage
* By the pool with Sam 2 - Aroused by Sam's dream
* Relaxing with Ellie and Julia by the pool 1 - Julia caressing her inner tights
* Relaxing with Ellie and Julia by the pool 2 - Julia slides the ice cube on her neck
* Relaxing with Ellie and Julia by the pool 3 - Julia uses the ice cube to circle her areola
* Relaxing with Ellie and Julia by the pool 4 - Julia teases her nipples through the fabric
* Relaxing with Ellie and Julia by the pool 5 - Julia teases her exposed nipples
* Spending time with Alyssa - Excited by how far Zack's massage could go
PD 12
* In the spa with Vicky and her boyfriend DeShawn 1 - Aroused by watching the two having fun in the sauna
* In the spa with Vicky and her boyfriend DeShawn 2 - Aroused by watching the two having fun in the sauna
* Visiting Larry Morello in prison - Aroused by Larry's fantasies
* Aiden’s 3rd task 1 - Excited by imagining to undress in front of the class
* Aiden’s 3rd task 2 - Excited by the boy's anticipation of seeing her exposing the left breast
* Aiden’s 3rd task 3 - Aroused by Aiden squeezing her breast while posing
* Nathalie's special service 2 - Nathalie running her fingers on her breasts
* Nathalie's special service 2 - Nathalie drawing her nipples out with her fingers
* Relaxing with Sam in the hot tub - Sam sucking on Sophia's swollen nipple while she talks to Liam
So there are 8 events directly related to that out of 24 (+Amber).
Obviously we can notice the escalation and the change of pace even in this regard for the last 3 PDs and all those related to the nipples happen in this time frame, starting with the fashion show. That's why we probably had that impression.
Interestingly, nothing happens during the 2 previous tasks in the classroom, we'll see if that somehow changes in the 4th task.