Still, lightning is a stream of electrons, and a stream (and not only) of ionized (an ion is an atom that has lost one or more electrons) and highly heated particles is called plasma. Fire is heated molecules or atoms – but this is just a tangent.
Of course, it's hard to disagree with Ganymed about Portal and Clone. On the other hand, everything happens in space, so why make all the magic from one school? It wouldn't be interesting to play, so the developers spread things out to balance the game.
By the way, in Heroes 3: The Restoration of Erathia, Portal is actually in the Magic of Fire. :D :D :D