In Idaho there is no law saying that you cannot carry on campus. State preemption allows the University of Idaho (due to it being a separate political entity) and the other schools in the Idaho university system to make their own rules regarding firearms possession. For students, faculty and staff it is banned for all and you will be expelled or fired for doing so.
There is nothing legally they can do for a non university affiliated person with regards to carry, OC or CC other than call the cops and have you trespassed. When I hit 21, I concealed on campus every day for the next 2 years, only a handful of my friends knew and most of them carried too. I still conceal when I go visit UI.
I can guarantee you that if you do OC on campus, people will freak out, call the cops and the cops will respond in a very inappropriate manner. Think SWAT team.
The part regarding the colleges is at the bottom of the page.