I want to make it so that interacting with a character, let's say a "teacher", will allow people to earn a skill if they have enough experience.
So, how can I create an event that boils down to "if character's experience >= 100, then skill added".
Learning a skill when condition allows
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This should do the trick

You can also have a Common Event that reads each Actors EXP on a Parallel Process (I don't know much about Parallel Processes, but it shouldn;t hurt to do that) -
If you don't know much about them, how do you know it shouldn't hurt?You can also have a Common Event that reads each Actors EXP on a Parallel Process (I don't know much about Parallel Processes, but it shouldn;t hurt to do that)
In fact, running parallel processes when you don't need them WILL hurt. It will cause your game to lag - the more parallel processes you have running at once, the more it will lag. So if you don't NEED them, don't use them.
If you know EXACTLY when you need this information (ie - when you interact with the "teacher" event), then you don't need a parallel process, as you can just do the test then and there. And since EXP only changes when you battle or when you manually adjust it, it's not going to change randomly on the map, so you still don't need a parallel process (which does its stuff 60 times per second). -
The event setup given by Bloodmorph is correct. But like Shaz said, it's better not to use a Parallel Process for this kind of thing. An event that you can interact with to run the check on your EXP and give you the appropriate skills will take far less of your system resources than a Parallel Process.
If it's important to be able to run the event from anywhere, you could use an Autorun Common Event, by setting a switch at the beginning of every battle, and having that switch be a condition for the Autorun Common Event that checks your EXP and grants skills if appropriate, before setting the switch back to OFF so it doesn't run more than once per end-of-battle. (You could even do this for a PP Common Event, but that's dicey because it means other processing can run while the event is running - which, in rare cases, could cause two of your events to try to do something to a character at the same time and cause bizarre behavior. Autorun is usually the better option for this kind of thing. PP events should only be used when something needs to be done while other action is happening outside the event.)
One thing that's maybe being overlooked in this topic - you can already learn skills by leveling up (with EXP), on the "Classes" tab of the database. Maybe there's a reason you don't want this to happen automatically and you want the player to have to visit the teacher to learn the skills, or maybe you're trying to separate levels from skills - but if that's not the case, yeah, "Classes" is the easiest way to do it.