I'm pretty sure "Summon Skeleton" is very different from actual necromancy. Actual necromancy involves getting a corpse and binding a spirit to it, not just calling one out of nowhere. Conjuring undead is distasteful, but not as abhorrent as using actual people's bodies.
I'm pretty sure that's correct, conjuring undead does not require the player to first acquire a body and soul for the proccess, which what we've learned about necromancy indicates necromancers need to perform their art, and the undead summoned by the player only remain by the players' side for a short period of time, whereas undead reanimated through necromancy seem to remain indefinitely, until they are destroyed, thus, if undead summoning spells are meant to represent necromancy, they're a VERY poor implementation of it, and in any case, if they were necromancy, than the Mages Guild in Morrowind would be openly breaking laws, since necromancy is illegal there, and the one in Oblivion would be quite hypocritical, since as of Oblivion, the Mages Guild had banned the practice of necromancy, and in both games, guild mages sell undead summoning spells.
I wouldn't expect to be able to join the necromancers, though, because that would kind of require a proper mecromancy mechanic, and I'm not sure if Bethesda wants to create an entirely new form of magic just for the sake of one faction.
But we really can't be sure what joinable factions are in Skyrim, we know the Dark Brotherhood is in and I'd assume they're joinable, and we know that the Mages Guild had split into two factions, with some speculation as to their nature, but little confirmed information, but this was in the time of Infernal City, over a hundred years before Skyrim, so it may be that the situation has changed, even if both factions still exist at the time of the game, there's no guarentee that they're both in Skyrim at all, let alone both are joinable. What I can say is that I believe that at the very least, an equivalent to the Mages, Fighters and Thieves Guilds will be in the game, but they may not be the same groups we remember from past games, 200 years gives Bethesda a lot of time to change things around with factions, and we really can't say what they'll do, I just think they will want to at least cover the basics. I'm hoping we can also choose to join one of the factions in the civil war as well, though Bethesda might have that as part of the main quest instead.