WMatsui couldn't get the guy, so in the end... they got each other? Lol, it's confusing.
I thought it was clear that the PV's not strictly in chronological order.
Rena was alone by herself for who knows what reason, relevant shot of the shoes. Jurina breaks that isolation, they become friends.
Jurina becomes smitten with the teacher. For some reason Rena is friendly with the teacher. PV does not make it clear whether or not Rena likes the teacher romantically.
Rena either:
1) feels lonely again because Jurina is ditching her for the teacher
2) jealous because she likes the teacher, too
3) is sad for Jurina because she knows her crush is a lost cause
Both 1) and 2) are compatible with 3), no matter her intentions towards the teacher, Rena is definitely sure that Jurina is set up for heartbreak.
Jurina witnesses that Rena is friendly with the teacher, either asks her to help her confess, or asking her to back off on the teacher. Rena agrees.
Jurina's confession fails, Rena comforts her, mirroring how Jurina pulled her out of her own isolated funk in the past.
At some point in the future, Jurina is over her crush, and witnesses a couple kissing, isn't bothered by it, and in fact relegates the tale to her homies back at the dorm, at this point Rena is accepted into the rest of the school as well, seeing as how she's integrated with the rest of the students.
Going out on a limb here, but given the rest of the PV, it's even likely that the couple Jurina saw was the teacher, and this demonstrates how she is indeed over her crush.
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First time through I completely missed Manatsu's storyline.
Bias goggles are powerful, man.
Also, I like how only Manatsu's guy is clearly a lefty. Everyone else seems to be regular right handers.