The spell's school is whatever the duplicated spell's school normally is
The basic use of this spell is to duplicate any other spell...
Duplicate here mean to "make or be an exact copy of". Note that the wording says the entire spell is duplicated, not just its effects. An exact copy of a spell means that it is identical in every aspect, which includes school.
If wish was intended to only take some of the characteristics of the spell and not others it would say so. In fact, it does say so, but only for two specific things.
Wish specifically indicates things that are not duplicated from the spell
You don't need to meet any requirements in that spell, including costly components.
Wish tells us that only one thing is different casting a spell using wish versus casting it normally: requirements of the spell do not need to be met.
...The spell simply takes effect.
Also, it is implied by the wording "the spell simply takes effect" that wish also overrides the casting time of the duplicated spell.
Since no other exceptions are noted the spell should be treated exactly as if the spell had been cast normally including duration, range, school, etc.
Wish only lasts for an instant
Wish is an instantaneous spell which means no matter what it is gone the instant after you cast it. But say you used wish to cast a spell, say geas, on a creature using wish. The instant after wish is cast the only effects remaining are going to be of the spell it is duplicating not wish itself.
Wish conjures a duplicate of another spell
Essentially, it seems that using wish to duplicate another spell would result in the casting of a conjuration spell with an instantaneous duration which then results in another spell taking effect with all the details that that spell has (including school). Wish conjures the other spell into existence and then disappears leaving only the conjured spell's effects.
You will still get stress
Jeremy Crawford has clarified exactly this in this tweet:
Wish spell: (1) Duplicated a spell of 8th level or lower? No stress. (2) Did anything else with wish? Stress. #DnD
A normal reading of the rules you already quote also specify "any effect other than duplicating a spell" so I think it is very clear that this is the case
Best Answer
Wish lets you ignore the requirements of:
Requirements, unfortunately, is never really given a game term definition.
So as with all words that don't have a game term definition, we should attempt to use the standard english definition.
Using that, we can say that anything is a requirement if it prevents you from casting the spell.
So with that in mind, here's the justification:
[Level] [School] - If a spell is not normally of the right level or school for you, you cannot cast it. This is a "requirement".
Casting Time: x - If you do not spend the amount of time specified, you cannot cast it. This is a "requirement".
Range: x Having nothing in range does not actually prevent you from casting a spell (for example, readying) - it prevents you from targeting it. This is NOT a "requirement".
Components: x, y, z (special) - if you cannot provide the proper components, you cannot cast the spell. This is a "requirement".
Duration: x - duration has nothing to do with whether you're capable of casting a spell. This is NOT a "requirement".
Concentration - When casting a spell normally, you can choose not to maintain concentration. The spell will end, but you are not prevented from casting it. This is NOT a "requirement".
Classes: x - If a spell is not on your prepared or known spells, you cannot cast it. This is a "requirement".
Description: [Effects] - Effects can only take place after casting is completed. This is NOT a "requirement".
Spell Slot - If you cannot provide the right spell slot, you cannot cast a spell. This is a "requirement".
Specific features (in addition to Wish) can negate some or all of these requirements, or even add requirements: see Subtle Spell or Ritual Casting