While I understand that Mage Armor (and more recently the Greater variant) has historically belonged to the Conjuration school, I believe it would make far more sense for it to belong to Abjuration.
There are a few other similar spells within the Abjuration school that do similar things and are not classed this way, such as:
Shield (+4 Deflection and Force DR)
Protection from Alignment (Deflection AC)
(Lesser) Globe of Invulnerability
Likely Mage Armor would need to be removed from Shadow Conjuration as it would no longer be a part of that school, but from a lore standpoint this would bring it in line with other defensive magics.