I noticed that the only thing which makes necromancers unique (TES4) are their summoning of undead and their robes, does that mean that summoning undead is considered a necromancy, also is necromancy tied to other magic schools or is it a school on it's own?

Almost everything discussed http://www.gamesas.com/bgsforums/index.php?showtopic=1003460 is of relevance to you. It starts getting side-tracked into Daedra later on, but it serves to cement the point that there is a clear lore line drawn between summoning temporary undead and keeping permament undead around.