@Dogalicious Maybe it's because Yamada is so physically mature that you made this mistake, but yeah, they are all silly, childish, naive middle schoolers. If this was a classroom of high schoolers, it'd be conversely weird how few of them have love experience. They're in their second year of Japanese middle school, which is the same as the last year of middle school in American schools.
By the way, "second year middle school" aka "chuugaku ninensei" aka "chuu-ni" is also the basis of the word "chuunibyou" which refers to how kids like Ichikawa have silly delusions in their second year of middle school, like about how they're a dangerous guy with the mind of a killer (because serial killers are "cool"). It's part of how kids their age try to find their own identity.
@Suzushiiro I don't think Serina caught on, unless I missed something. Moeka is just good at noticing things like this, way more so than most middle school second year girls (this was also true back when she knew immediately that Pick-Up Pai just wanted Yamada when he invited Kobayashi out). I agree that Kobayashi is a bit oblivious/childish, but don't think that Serina is much better fundamentally.
P.S. If anyone is having trouble remembering the girls' names, check chapter 33 page 18, I have it saved on my hard drive for reference.