In the teaser Sophia is wearing a bra. I believe, once Dylan is at home, this event Dylan/Ellie takes place in both cases: 4th taks or not.Just a thought. If Sophia leaves school without wearing the bra and the event at Morello's also takes place topless, it would be possible for Sophia to start the tutoring sessions without wearing a bra. The boys would then not need to persuade Sophia to take off her bra at all. This would avoid the whole discussion - I'll show you mine and you show me yours. It would then be enough to open the blouse a little and Sophia could tell herself that she is not taking her clothes off. In a way, it would be a smooth transition to the next level of corruption.
But, that " In the meantime at home... " conveys the idea that it happens during another event
Unless the bra is in her bag, Sophia has to return home to put it on. It's possible that Sophia is the cause of the split in this event.