I'm not going to lie, the word 'academy' made me think modern as well haha. The whole 'pay to learn' thing was what I was sort of leery of when it was brought up, but I wanted to learn a bit more about the base concept before bringing it up. Even the Lhavit Towers don't just allow people to walk up, pay, and get in to learn magic; there's a process that PCs have to go through in order to earn that right. Straight-up just paying to learn magic wouldn't sit right with me, I don't think; the struggle of seeking it out and earning it is, in my opinion, part of the fun of it.
I like the idea of really reclusive, secretive groups. Maybe these groups are concerned about how most places dislike magic, and are worried about knowledge dying out, so they actively seek out people they feel would be worthy of continuing their djedline? Maybe some of these groups even trace back to during the Valterrian, when mages began putting together whatever resources they could while sheltering underground in hopes that it would be preserved and passed on.
There's been mention of rival academies, which I think is a cool idea, but I also wonder now if there might be a more insidious, sort of opposite version of them. Would it be possible to have a group that has a mindset like Sunberth, where they despise and distrust magic, perhaps blame it for the Valterrian in some way, and as such they are trying to hunt down and eradicate these academies to stop it from being documented/passed on?
I just woke up so I'm just sort of giving my first thoughts/spitballing off what other people have already said here, but I'll definitely be thinking about this idea as it's really cool, and hopefully when my brain wakes up more I can come back with a few more suggestions.