God, I really hope a poly happens between Sayo, Rinna and Mei. I mean come on, we had a chapter where Mei meets her teachers, then proceeds to ask them about being in a relationship with more than 1 person, then on top of that Rinna seems to be falling for Sayo while still having feelings for Mei.
Hasn't the whole Rinna x Sayo thing been dropped? Like the author teased it but since then has gone more of the love triangle/harem approach?
It might be dropped, but not necessarily. If I were to write this I would plant the idea of Rinna x Sayo early on (like in the tease) and then spend time exploring both Rinna and Sayo as characters as well as their feelings for Mei and only then showing them falling for each other.
I can't confirm it's gonna go that way, but it is how I would do it and this definitely could go that way. I certainly am hopeful
But that's the thing the story is taking the time to talk about their feelings of Mei, not their feelings for each other. Take everything we've seen with Rinna and how genuine she seems about loving Mei. Wouldn't it feel ooc of her to suddenly say "I think I love Sayo too now"? Yeah if done well it could work but then you run into an issue with Mei. Mei isn't in love with anyone and believes she can't love someone, though she wants to. Imagine if her two friends ended up loving each other. Wouldn't she instead spend her time getting them together rather than wanting all 3 of them together? I feel like them falling in love with each other would hurt the story of Mei finding her true love. To me I don't get why people want a poly relationship considering right now Mei doesn't want to date anyone and when she does I feel like that person has to be special. For example would Bloom Into You have the same impact if it had a poly ending?
I'd like to point out:
spend time exploring both Rinna and Sayo as characters as well as their feelings for Mei and only then showing them falling for each other.
What I mean is that while the possibility of poly should be planted at the start (which the teacher as well as the accidental kiss did), any feelings that Rinna and Sayo might develop for each other should only be shown after the series properly talks about the individual characters as well as Rinna's and Sayo's feelings for Mei.
Either way, whatever happens determines whether poly would be satisfying or not. At this point it's way too early to say one way or the other. All I can say is that I'd prefer poly (because a. there's way too few poly yuri manga and 2. I think that Mei learning to love trough an unconventional relationship would be really interesting), but wouldn't mind mono either.