Juvenile means it's about young people. Not prison.
I know, I mean the fact that it's high school, but:
Not at all. He's wearing the uniform for P4's high school.
I missed that, so I guess he isn't.
The new characters from Persona Q are also pretty cool, I'd be ok if the three of them were from Persona 5, but it's fine anyway. Excited to meet the new characters.