Basically they were trying to scare 12th gen. The power went out, then Yoshizawa and Tsuji came in and there was a bit of chaos getting organised again. Tsuji had made flavourless ohagi, and the girls basically had to lie and say it was good, even when it wasn't (the hosts were impressed by Ogata's reaction, I think).
Yoshizawa then asked everyone who their admired senpai was... of course trying to get them to say it was her. Maria was the only one who said otherwise (Sayumi), and then started crying. (Worth noting that Yossy said she felt really bad afterwards and seemed concerned about Maria.)
Then when they were on stage, they played scary sounds into 12th gen's ear monitors (a male voice saying "help me"), which caused them to freak the fuck out. After the power went out
again, the original plan was for the older members to pretend to be dead, and 12th gen to scream and run around. But they just went with it and played dead as well

. Then Kei came on stage in that outfit, and the original idea was to scare 12th gen even more, but they were playing dead and just kind of ignored her.
Also, at the end of the segment, Maria said thought the ohagi were flavourless because that was the way Tsuji liked to cook them (as in, she was a boring cook).