So from the scene with Star's parents at the beginning of Baby, with River arguing not to pull Star back to Mewni because it's still in the school year, gives me the strong impression that the season finale, set around the last day of school, will involve Star possibly being sent back to Mewni. Probably not permanently since obviously there's an entire season after this, but more of a threat than it already was this episode (and it did get pretty heavy this time toward the end).
As of right now my dumb speculation is that Star either fucks up Marco and Jackie's relationship in Just Friends (whether causing Jackie to initiate the breakup because of how close Star and Marco are or just making things too awkward for Jackie and Marco) or gets yelled at/chastised for Marco for being a clingy third wheel.
In Face the Music, set on Star's "song day," which seems to be a rite of passage or something on Mewni, the tension between Star and Marco from the previous episode causes Star to severely fuck up whatever she's supposed to do and draw herself and Marco further apart (the latter probably influences the former too). Star's parents decide she has to leave Earth.
Star is crushed about this in Starcrushed and no longer feeling so close to Marco, and so some of her female friends meet up to comfort her. As we know, she skips out on the Diaz end-of-school-year bash and shit goes down.
Just my guess. They seemed to be pushing "can't send her back when she's in school" pretty hard.