Noriko is F because hers are clearly visible. If she was E or under it's possible to hide it under your shirt but F got that cleavage...
Naoto is D and I can confirm that thanks to the golden ending. If the manga is relevent, there's that too.
Rise got that side boob, frontal view shows not much, but judging from the side boob I'd say she's C.
Yukiko was eady to tell thanks to the Midnight Channel. Also, she implies that hers aren't big which encourages Rise to tell her not to worry. I don't think she would've said that if hers was bigger. Maybe she would've said, "oh, but your are bigger than mine so you shouldn't feel upset" but she didn't.
Chie is clearly flat chested but I wouldn't call her a AA since she got something showing in her bikini. I'd say she's A with that.
The official Drake, Toro, and Sir Daniel of the PSABR boards.
While watching the scene where they go to the beach, Yukiko seems the flattest. We also can't use the midnight channel as a way of figuring out anything, considering Rise's Midnight Channel appearance was bustier than the real her.