I agree that it's interesting to have many differently aged viewpoints. And I think it's rather important for Akira to be at least 30 in this story because, although still illegal, if he was, say, 22 (sort of Imani's age) - I feel like he'd be closer to a gray area age-wise. No disrespect to early twenties folk, but sometimes when you talk with certain 20 year olds there's barely a difference than when talking to a 16 year old.
Obviously the teacher-student thing is simply a no-no, but if Akira was 22 and he went out with a 16 year old it'd be different than what it is. Cause I can hardly imagine how a 30 year old would go out with a 15 year old when even having a conversation with one can be hard at times (but I can imagine a 22 year old doing it though). Tbh, regardless, I don't even think the sex would make much sense cause they'd simply wouldn't be there mentally... Even Imani and Kaori would already be stretching it as dateable in my eyes (age-wise).
Also important to note that none of this would apply anyway because, as Yumi points out, Akira isn't dating a student - there's no gray area (other than the supernatural one): he's dating 20 different people and is basically one step away from a sex cult.