Ok Dengeki has the first small details in their new issue.
The setting is indeed High-school like in the other persona games as well but the tone is said to be completely different. What ever that means. Maybe more realistic in terms of the Japanese Education system.
The theme is about the emotion of joy after overcoming trials therefore the chairs and chains in the trailer.
They say that Persona 5 is a RPG with the message of expressing the emotional catharsis in the shout of joy made after overcoming your trials and finally being set free of your chains. I would say that the theory about it being about pressure and stress from the Japanese school system is probably a very good guess
Personally I am a bit disappointed that it is not set in a university setting but given the huge sucess of Persona 4 it would be a bit naive to think that this will happen.. None or less I am very interested to see the tone shift. If it is indeed more like the real japanese school and education system it could be very stressful and also grim. But we will see I guess.