The happy ending of Hope Arc was only temporary.
The first and last class of the new Hope's Peak Academy, most of which being outstanding youths that worked in the future foundation and fueled by their hatred of despair, used their talents to do everything they could to find out where the remnants of despair fled off to after their message against the world and how the remnants came to be. They got more then they bargained for, discovering the truth of how Hope's Peak, and talent in general, lead to the tragedy.
They broadcasted this information to the world, and the world ultimately decided that Hope's Peak needed to be shut down and the remnants must pay, despite everything. The class, in response, left to Jabberwock island. They were lead by the Ultimate Lucky Student and soon-to-be new Ultimate Hope, someone who was unlucky enough to meet all the remnants yet lucky enough to survive their torture. Someone driven mad by what they experienced and had trouble befriending their classmates due to every relationship they had during the tragedy ending in disaster. Someone whose body was so mutilated when they washed up on the gates of Hope's Peak academy that they now look more machine than human.
As great science is built on the shoulders of giants, and due to all the knowledge the class they found out through research and Future Foundation awareness, class 77-B couldn't stand a chance. They found a way to eradicate Komaeda's luck and Kamakura's existence. They massacred Class 77B: They were melted alive. Frozen and shattered. Maimed and trapped in a burning building. Hacked into bits. Suffocated from toxic gas as they cried for the ones they lost. The Hell Queen of Novoselic was the last person to be killed by the torturous devices she made. The Ultimate Filmmaker's broadcasting of such events let the whole world watch and cheer not in despair, but in hope over such evils being hushed away.
As for the survivors of Class 78, they ultimately survived unscathed, but not without some difficulties. For once they were on the wrong side of Hope and Despair, as they tried to halt their students discovering the truth and broadcasting it to the world. They ended up having to make a tough choice: either accept the academy they rebuilt being shut down as their upperclassmates were executed, or go down with them as traitors.
