If the Oblivion crisis shut the door on Daedra, why can I summon a Dremora Lord so easily? And are atronachs deadra? I thought they were magical constructs, but banish daedra works on them.

Yeah we get allot more undead. But that should be separate school, IMO. Necromancy. If you need a corpse first, you haven't conjured it. It was already there. You gave it some kind of pseudo-life temporarily. That should be necromancy. Not conjuration.

But I'm with Meridia on this subject. Undead are fake, dead, nasty, repulsive and dirty things. Daedra, though evil and arrogant, are alive. And it makes conjuring and subjugating one to your will, more fun. Undead things are just robots. Really gross robots.

The Oblivion gates were closed meaning Mehrunes Dagon or other Daedric princess can not start on invasion on Tamriel again. They and all other Daedra can still be summoned. Only for a more limited time. So that makes total sense.

AS for Necromancy, essentially all schools of magic are just inventions of some early mages guild people who thought it would make magic easier to teach to most people. You could put destruction spells in illusion and it would still make sense...

Indeed, reanimating a corpse could be

Restoration because you inject life in a lifeless corpse.
Alteration because you modify the corpse for it to be animated.
Mysticism because you call on the soul of someone to inhabit his corpse again.
Conjuration for same reason.

School of magic are just name arbitrary given by the earlier mage guild.

And atronach are indeed daedra.

No...the names aren't arbitrary. The names were chosen to teach, so they tried to make sense with them. You wouldn't call destruction magic "Fred". or Illusion magic "Zip-Gorch". In Morrowind and Oblivion I had a tendency to call Conjuration, hasenpfeffer, because I used it to summon things for soul gems. Good to eat. And that was rather arbitrary, I suppose. Elmer Fudd shouldn't have that much power over my imagination.

But I do see your point. How the fictional College of Mages chose to describe the fictitious schools of magic is subjective. My own subjective opinion is that reanimating corpses is better described by the words necromancy, necromancer than by conjuration, conjurer. That seems more logical to me. But hey, what the hell, I didn't make the game. And it's a great series of games anyway.