"Did you know I died?"

Or something like that...

Menma being a ghost would explain why she walks around barefooted.

Would also explain why she looks so much younger than the rest of the cast, but, until I see a body, it sounds more like a metaphorical statement than anything else. Or does she specifically say "killed" or such in Japanese?

She says something like "Even I know...that I'm already dead."

Jinta: Back then, that summer was when everyone followed me. Yes, even on that day.
-----
Meiko: A friend?
Jinta: Of course not. It's not like anyone cares about me.
Naruko: It doesn't really matter if you (Jinta) came or not. But you're such a disgrace!
Atsumu: What's with that "Menma"?
Jinta: Ah!
Atsumu: Are you... still saying that? (as in,"Are you still using those childish nicknames?")
Jinta: Do you.. do you get it now? Everyone's changed. No, aren't I the one who changed the most?
-----
Meiko: Wait, Jintan! Jintan!!
Jintan: When you're around I keep remembering all the bad things. It's annoying.
Naruko: I'm the real disgrace...
Atsumu: Skipping school and calling out to Meiko Honma; Are you retarded or something?
Jinta: !!!
Meiko: Jintan! Yukiatsu, you dummy!
Chiriko: People change when they get to know Meiko Honma. You're no exception, right?
Meiko: Menma knows... that Menma... that I'm dead. I know that.
-----

* Sounds like he literally said "scientific/technical name". Considering the show's title it's safe to assume that they really did find an unknown flower at some point (when they were kids?) and Jinta chose to name it after the apparently deceased Menma. Scratch that I was wrong orz.

Sounds to me more like Yukiatsu was saying "Not going to school and (calling out/speaking) Meiko Honma's name. (Are you sure your head's on right?)"

I definitely struggled with that line. It sounded like he said ”学名数に本間芽衣子の名前呼んで” which made sense when I tried taking the title into context. Your skipping school part also fits Jinta's hikki status but what was said didn't really sound like it. Then again I'm still an amateur so you may be right.

It was 「学校も行かずに本間麻衣子の名前を（よんで*）」 which seems to fit with what Yukiatsu was saying earlier in the PV (~"What do you mean "Menma"? Even now, are you still saying that kind of thing?")

*sounds like it could be よんめ but that doesn't make sense...

It's like the whole thing about make my wish(meiko) come true is the last thing that she want to do before she goes to rest in peace.maybe she thought that if everyone come back and play together again.Jinta will smile and cheerful like before. But reality is the other things. And if the wish come true and meiko will be truly disappeared in that time.That sound sad enough.:(

"I know! At least I know that I'm already dead!"

I sure hope that she's not dead. Maybe she's in a coma and her spirit is wandering around or something like that. 'Cause, though the anime hasn't even air yet, I like Jintan x Menma already.
And there is something about the orange twin-tail girl that I dislike, for now anyways, because she seems to be an annoying Tsundere character. Don't misunderstand me, I like a lot of Tsundere(s), most of the time they have this cute side that just makes you go all 'aw'. But Naruko (or whatever her name is) looks like a b*&chy Tsundere instead of adorable one.