UN... well, to be exact, Saruta attacked the school on the same day that Kuro and Hibi went to that other high school to supposedly attack. Well, turned out that Yamada, the booger-eating girl, was actually another ninja, but from UN.
Hibi got totally defeated without putting up much of a fight, and Yamada showed resistance towards electricity. Before fighting Kuro, Kuro summoned a sword through a drone, using a ring with GPS in it. Yamada also summoned a sword, but in a much fancier way, another showcase that UN had better technology compared to NIN. Despite of Marisihiten's supposed blade-proof ability, Yamada's sword managed to cut Kuro's side. Kuro only managed to take Yamada's nose. Just like that, Yamada emerged victorious, and Kuro was killed. Hibi couldn't believe it, Yamada then proceeded to rip Kuro's face and throw it away. Yes, Kuro died.
On the school, Saruta began his attack. The whole school was in panic, it was a total chaos, so I'll just write the result: Kato used CUBE without permission. Onikobe got killed. Suzuki got one of her feet cut. ESCAPE was used in full force, destroying the whole school to erase the evidence. Eita, Noguchi, and if I recall, the occult girl managed to survive somehow.
Kato was then put on a trial because he used CUBE without proper authorization and let Onikobe get killed. He was later put on that "Together with the Elderly" show, with Nijiro Kumogakure/Rainbow Hound himself as the executor. There seemed to be intervention from UN, showing that they somehow managed to infiltrate the place. He managed to survive. Rainbow Hound died with the explosion from UN's bomb. Kato defected to UN.
Now you might be wondering what's the deal with Kuro being dead. He's replaced by Jyuuro/Juuro/whatever you want to romanize his name as (from 9(九) to 10(十)). He's actually the guy that you see in MangaDex's page of the series, on the cover. If you see on the cover page, the covers for volume 8 and 9 have Kuro passing the baton to him. He knew Saruta and Nekohira from their days in shinobi school.
From this point on, I really stopped caring and had become bored of the series, and thinking of saving up a lot of chapters before reading it again, so honestly, I wasn't paying attention at all to the details. Jyuuro seemed to be able to speak in cat language, too. We later got to see the foreigner guy and another new foreigner. That's it.