If you own Realm of Magic, you can teach other Sims magic after reaching Level 3 of the Magic skill. You can train or instruct Sims that are not of higher rank than you. By doing so, you’ll also level up your friendship with them.
You can also turn other Sims into spellcasters provided that you’ve reached the Grandmaster level and learned the Ultimate Practical Spell yourself.
However, you cannot teach other Sims specific spells. Only Sages can do that. Those Sims will have to learn how to cast specific spells from a sage or study spellcasting by themselves just like your Sim did.
They can discover various spells through practice. However, they need the help of Sages to learn the ultimate spell of each type of Magic.
As a quick reminder, there are three Sages in the Realm of Magic, each specializing in a different type of magic: Practical Magic, Untamed Magic, and Mischief Magic.
You cannot become a Sage yourself even if you manage to master all the levels of magic. However, you can do two of the things that Sages can do.
You can train less experienced spellcasters and make a normal Sim into a Spellcaster after learning the Ultimate Practical Spell.
On a side note, there’s a lot of lore related to Spellcasters in The Sims 4: Werewolves. Spellcasters initially created werewolves to fight against vampires.
Things took an unexpected turn when they become too greedy and wanted to experiment more with the power of the Moon.
If you ever get bored of being a spellcaster, you can stop being one by asking a Sage for the Ask For Rite of Dissolution.
Long story short
You can train less experienced spellcasters in The Sims 4: Realm of Magic but you cannot teach them how to cast specific spells. You can only help them improve their Magic skill but they’ll have to put in the effort to unlock specific spells on their own or learn them from sages.
Happy Simming!