Is that why he suddenly disappeared? Time-line-wise, I mean?

No. He disappears after the first day of high school. They're still in middle school.

Ah, ok, thank you! I wasn't sure at all anymore, when anything was supposed to happen XD

It's been a while since the first chapter and some people forget, so no worries. And no one said Jian Yi was only going to disappear once :P Maybe he'll be rescued by He Tian's bro before anyone else notices he's been kidnapped.

Actually, now that I think about it maybe that's why Jian Yi's dad abducts him: because Jian Yi is getting into too much trouble, so his dad takes him in to protect him.