For anyone who doesn't get Yamada's (maybe subconscious) motivation to interfere with Sekine and Ichi's studying, it was made pretty explicit in chapter 125.
There's a lot going on in this chapter, and one of the reasons why I think the writing in this manga is superb. They're both being shitty to each other, like middle school girls often are, but Yamada is the genesis of this by letting her insecurities and subconscious lack of trust in her boyfriend and her best friend get in the way of what's best for both. Sekine has been doing a lot for everyone on this trip, and Anna's actions don't match her praise for Sekine in this chapter. In Sekine's mind, you can't be shitty and annoying to me when I'm trying to study because you're jealous and then tell me you think I'm amazing. I'm not saying Anna is doing this on purpose, she's young and immature and probably doesn't even realize she's doing it, but she's still being at best inconsiderate of her friend.
And that's okay! It's good writing to see characters overcome their flaws! It's clear Sekine also has a lot of pent up jealousy towards Anna, as well. I dunno, this is just my interpretation, but I really like the story and think it's very well done.