I agree that Ranna is the final boss and the crew is preparing for that eventuality, but something about the scene felt off. First off, where was Rokudou in there? Second what's the ex-clown's end-game? He's clearly not gonna support his boss's plan.
My first guess is that something bad happened to Rokudou causing Ranna to rage out, but the crew tried to stop her because they think Rokudou has a plan or some other reason. And when it comes to Rokudou, Ranna loses all reason.
My second guess is it's staged. The organization knows her power and they kidnapped Rokudou to control her. But instead the crew makes it look like she slaughtered them only to surprise attack them when they arrive at the school. I know what I said before, but Ranna is not interested in fighting anyone at the school. Or interested in anyone in the school period.
Plus, the start of the arc made it clear it was about Chiyo. Meaning we're getting a new batch of girls for the year and Chiyo is just the start for the second year arc. The Sakura Organization is part of a mafia, so that means we'll most likely be moving up to actual Yakuza gangsters, guns and all.
Lastly, I don't think we're in the last arc, because we still don't know who Ranna really is. She's shown to be closed off and violent, but it's clear there's a reason for that. Not only that, Rokudou is still learning the limits of the spell, which means he may even learn how to turn it off and on and it may not actually go away.