The High School setting is a tradition dating back to the series spiritual origin Shin Megami Tensie If and I doubt they would move away from it. Hell they sidestepped making Strange Journey SMT4 only because it didn't take place in Tokyo which every main line SMT game has done before. This staff is a stickler for tradition and I doubt they will change it for anyone.

Technically the high school setting works too easily for the setup and lots of things can be done with it still. College in Japan, from what I've heard, is not like it is in other countries and is much more relaxed and subdued. Not the type of place good for social links and side activities like high school is (which is closer to colleges outside of Japan). There are reasons why anime doesn't dwell on college settings.

Besides Persona 2 showed its possible to still use a high school setting with an older cast as well. I still feel that Persona 5 will be a fresh start, with a new cast and the P4 Arena story arc may simply spin-off into its own series. This is probably a better deal for Atlus anyway because if P5 is a direct continuation from previous Persona games, it will continuity lock the series from new fans. Don't want to pull a Kingdom Hearts and force fans to buy a bunch of games (gotta make sure their the right continuity as well) just to understand the story to its full capacity. It would be more profitable to keep Persona 5 and the main line series into fresh starts and just create a parallel series that absorbs the new casts from new mainline entries and incorporates them into a larger epic.