I can definitely see Chika being the first to go, and I get what you're saying, that it could easily look like Sensei is creating a life with Chika that doesn't involve Ami, behind Ami's back.
Dating Chika, bonding with Chinami, the New home right near Sensei's secret apartment, etc, all things that could look like Sensei is ditching Ami for a new family. Being abandoned seems to be Ami's greatest fear, so it's something Psycho Ami could easily act upon. Removing Chika would be the logical option.
I also think Chika is someone that makes most of the other girls insecure without even meaning to. She's the oldest, and acts more "mature" as well as has a more "mature" body, etc. Things that I can see Psycho Ami feeling threatened by.
Chika is also one of the few girls who could actually pose a threat physically. Her, Yumi, Tsuneyo, Touka, Ayane, and maybe Noriko are the most physically dangerous of the class (unless Uta has a bow, then I'd add her). So, I'd probably take them out asap. Still, Chika being the most notably likely to take Sensei away from Ami would simply put her at the top of the list.
Then again getting rid of Yumi would be the easiest to get away with. The class delinquent disappearing would also remove an ally that could benefit Chika for when Psycho Ami kills her, not to mention, the smartest in class aka Nodoka probably wouldn't even care if Yumi disappeared. So, perhaps the first to go is Yumi, but Chika right afterwards. Albeit, I don't know if Psycho Ami would think that far ahead.
I also don't know how powerful Psycho Ami actually is, so, killing Chika or maybe anyone could just be a walk in a park, and all that's needed is some motivation.