That wasn't to stop everyone from leaving, it was to stop everyone from seeing a few <em>particular</em> people leave.
It was a sort of a crazy thought. If everyone in the Ultimate School Council was busy with the Amazing Himiko's magic show, it was sure that they would at least <em>notice</em> someone missing for 10-20 minutes. Unless maybe...
She turned her eyes to the person she hated the most in the world at that moment. The tiny weasel called Kokichi Oma. He could slip out of any room and no one would notice.
He didn't seem to like Kaito and Maki(come to think of it, was there anyone who Kokichi remotely liked in this Academy?) and with his weird jittery speed and excellent brains, he might have been able to trick/annoy Maki into getting stuffed into the storage cabinet, and ultimately, the washing machine.
There were only two major gaping problems with that theory. First, who was the second person? And second, what was Kokichi's motive. He had literally given them the hint on how Kaito had been poisoned only the night before suddenly changing his mind and killing him.
But still, unpredictability was Kokichi's whole thing. He could have easily seen the motive after helping them out and decided to kill Maki, the assassin who could kill all of them here instantly. She was an obvious threat to everyone, and anyone who knew that secret could easily decide to try and kill her.
Kaede decided on the more concrete thing--the thing that didn't require her to dig around the mess of Kokichi Oma's mess of a human mind like a carved pumpkin. Something that solid evidence might be able to help with.
"Could the curtain have hidden anyone leaving the gym?" she suggested, voicing her theory.
Kiibo stared at her. "Kaede, no one left the gym,"
"I know, but...is there a chance that you missed someone>"
Kaede offered. "Like, someone small and good at hiding...?"
Pretty much everyone seemed to get her gist now, and stared tat what was now the semi-lead suspect.
"Well, Kokichi?" Shuichi asked. "Did you leave the gym?"
"Huh?" Kokichi stuck his finger in his ear and twisted. "Sorry, did you say something? I wasn't really focusing--this class trial is getting really boring..."
"I said," Shuichi looked like if he didn't punch something right now, he was going to go insane. "Did you leave the gym during the Amazing Himiko's Magic Show rehersal?"
"Nope!" Kokichi answered a bit too quickly to be completely trustworthy, but it wasn't like any word out of the Ultimate Supreme Leader's mouth was trustworthy anyways. "I was there the entire time! Even the super duper boring bits!"
"None of Himiko's Magic Show can ever be boring, degenerate male! " Tenko struck a battle pose. "Apologize immediately, degenerate!"
"Wait," the look on Shuichi's face was deadly serious. "There's an easier way to find the truth and keep the class trial moving. Himiko, did you say the names of the acts you were going to perform all at the beginning, or did you say them at the beginning of each act?" he asked, turning his head sharply in the other direction.
