I finished the manga years before and I would have to say that at one point I agreed with your assessment with Rion. She really aggravated me. But. I actually started to like her towards the end since her character progress started to make sense. They were suddenly hurled into a place with no clue as to how they were there and what should they even do. I understood that while some of them would have moved on and was already concerned more about survival, some others wouldn't bounce back as quick as them. People have different coping mechanisms and some people actually adjust in a longer time span than others. Though in the end I was disappointed because even after years she still thought of the boys so badly (granted, they were) but still. It's been years, they should've talked already about boundaries and it seemed as if most of them haven't gotten over their childish tendencies. The end made it seem as if they still were children despite all that they've experienced.
The teacher (can't remember her name for the life of me), however I would like to defend. She was a language teacher if I'm correct. She isn't built for exercise or sport. Just look at her petite figure. I'm going to assume that you already finished this series already. If so, you would know from flashbacks that she strived to not only become a role model to her students but also someone they can depend on--- emotionally. And she was that for Yarai. She was his composure, his sanity, his conscience, etc. Since she stood up for him and kept on being kind to him despite his gruff and rough demeanor and over all personality. She kept on minding how he was despite everything and he was glad for that. She knows that too of course. She can read Yarai more than Sengoku could at times. Thus, she kept on following him because she knows that Yarai likes her presence even if she holds him back. In turn Yarai wouldn't let anything to her because she was -his- and all that that. That's why something kept on happening towards her. Yarai is generally indestructible but she was his weakness. The plot had to progress and if they can't touch Yarai then they would his weakness. She isn't useless. She's a great plot device actually.
Anyway, hope you liked the manga. Re-reading it a second time would help you understand some more details of the characters.
hmm... i only have to say that rion and the female sensei were there to make akira and yarai more reliable (i think?) it's the most logical (stupid) situation. the girl is weak, the boy is strong (somewhat) and he has to protect her from literally everything. in return they give the boys some sort of peace of mind and a dirty mind too... i mean they are boys.. even if they grow up for 3 years, the pervert side of them never change in my opinion.
p.s= i wanna point that i'm talking about the manga and not RL so i hope that u boys don't take it to heart. ok? ٩(๑❛ᴗ❛๑)۶

I'm at chapter 105 and I just have to say that I'm so so tired of Rion and that four-eyes teacher. Both of them are so useless. Rion thinks that Akira is dumb and a pervert and can't do anything by himself, ut who is the one that can't do anything? She is. Over and over and over again it always happens something to her and she can't do anything except for sceaming.
And that f*cked up teacher is worse. "Now you don't have to worry because I am here and I'm a teacher" is what she says everytime they meet new people but when Yarai is planning to go away and look around or do other helpful things she always runs after him and everytime something happens to her because she is so dumb and useless. I hate her so much. Why can't she just die??!!