Is Suzune Horikita a coward?

She knows Kushida is a pretty important classmate to make her objective of graduating from class A possible, not only with her physical abilites but also with her communication skills and intelligence. That's why Horikta doesn't want to expel Kikyo.
Later, the next seasons will prove my point, but don't worry, in this case, Horikita knew what to do.
About Yamauchi, he was bringing harm to class D, not only being an useless student, but also betraying his class to give information to Sakayanagi. Yamauchi isn't just being useless, he literally betrayed his class so he has more chances of survival.
I won't try to defend Kushida because I lnow she's evil, but it would be easier and better to eliminate Yamauchi at once.

@MKIkawa Kushida damaged class D several times and yet she remains unpunished? Then how was Yagamuchi harming class D? If we see it from the characters' point of view, he was just expelling a classmate who had remained anonymous throughout the school period, it wouldn't have been a loss for his classmates (precisely because he didn't know that Ayanokoji was the mastermind behind class D) in a test in which someone had to be expelled... from Yagamauchi's point of view, Sayanagy's solution in expelling Kyotaka could solve every problem without harming the class