But there are schools of magic that produce an effect...
Wouldn't an 'Enchantment' spell be considered something like a charm?
Or even the Command spell is an Enchantment spell...
What spells would not affect someone if they were immune to Charm?
Some races are resistant as well...
Nope, that's exactly how 5e does not do it. It's a change if you are used to 3e, for instance, since it relied on that sort of thing extensively.
The schools of magic, just like the creature types have zero game effect in and of themselves. They are only there for other things in the game to potentially refer to. So an undead creature has no immunities just by being undead. If a particular monster is immune to something, the creature entry will mention it, but nothing mechanically is implied by creature type.
It's worth pointing out that you may have an incorrect understand of charming in particular.
Nobody is immune to Charm or charms as you are envisioning it, since it doesn't exist in that way.
There is only a condition known as "charmed", that you will find described in the conditions appendix. Many creatures are immune to the charmed condition, but there is nothing that makes anything fall under that immunity other than by referencing the condition itself. The enchantment school has no inherent effect.
Being immune or getting bonuses against being charmed refers to the mechanical condition titled charmed and nothing else.
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