The new girl on the cover is a complete mystery. Maybe someone Naoi knows/knew? Her sparkler is hidden so it could be hiding that it's already gone out, meaning she might already be dead. Meanwhile Kokoro has a more modest sparkler compared to Naoi and Kurumi's much more flashy ones.
Naoi's abandonment issues on full display when Kurumi tries to go home on her own. But if you think about it, Kurumi's technically ditched her twice already. Both at the start and mid-way points of volume 2. Though I read a very interesting character analysis of Kurumi in Japanese that doesn't view Kurumi's behavior as mere indecisiveness. Rather the fact that Kurumi can erupt like she did in chapter 10 then try to carry on as if nothing happened in chapter 11 with talks about cram school etc. show a very unnerving aspect of Kurumi. That analysis deemed Kurumi possibly much more dangerous than Naoi, since Kurumi likely has much further she can fall. Naoi's essentially playing with fire by getting Kurumi to let loose on her true feelings.
I have to wonder what Kurumi's mother's mistake was. Her scars were apparently old even when Kurumi was young. I wonder if the mistake she thought she made was getting married or getting pregnant? Maybe a shotgun wedding and her putting so much pressure on Kurumi was to make her unhappy married life "worth it"? But right now her mom ran off to her grandmother's house and lied and said Kurumi was too busy with summer classes to go with her.
Kokoro is starting to spend time with Akane. By the way, there was a very small note on Kokoro's family's shop in chapter 9 that showed they were hiring part-time help. So it seems Akane was hired, which means Kokoro will be spending alot of time with her over the summer. So, yet another person in Kurumi's life who might be moving on. Which the text on the bottom of page one already heavily implied.
And finally, the phone call from the teacher at the end likely isn't good news. Should note that in chapter 1, the homeroom teacher was named "Horie", but here the homeroom teacher is referred to as "Hotta". They are very similar names in Japanese, so it's possible there's a naming error in the Japanese text. But if it is the same teacher that's already been shown, then I don't see the meeting going well. Their teacher was already mentioned by Kurumi to favor Isanuma. And now Kurumi's done a ton of things to make him look bad. So the meeting could very well be him wanting Kurumi to apologize to Isanuma and essentially try to shift all/most of the blame for the incident onto Kurumi, which would further her downward spiral.