Good question, since 'universal' is not truly a school. The way I'd handle it in play would be something like, "The spell/item, though magical, does not seem to belong to any particular school."
Cavalier: Well, what kind of magic is it?
Sorceror: ...this doesn't make any sense...
Druid: What?
Sorceror: According to my detection spell, this magic is 'vanilla'.
Druid: You must be doing it wrong. (casts a detection spell) ...Huh. Sure enough, 'Vanilla'.