Can't this be done with some Common Events? oxo Perhaps my line of thought isn't very optimized, but it might work... xD
A switch can be launched at the beginning to trigger this common event's parallel nature. It'll check if the actor has a certain skill, and if the actor lacks that skill, it'll check if actor player is at the right level to learn it.
...Now that I think about it, I should have put Erase Event at the end of the Common Event... xDD
Anyway, it can support multiple actors too, since all you really need to do is use that level variable again in another instance (Or use another one). Then at the end of the event you can just erase the common event, so it won't loop...At least I think it works like that. ._.
I haven't tested this and I forgot some of the specific things RPG Maker XP can do, so forgive me if this is a little wrong. But the concept should be pretty straightforward that way. xD
A trade-off here would be that it will only appear on the map. You'll also have to not let the actor learn any skills through the Database's Class thing, unless the skill was learned right off the bat.
If you're still bent on scripting, it shouldn't be too challenging, to think...x_X Should just be a new window/bitmap right?