Mmmm personally I prefer not to worry about things like money in an rpg like this. Majority of the characters are students that aren't even adults yet too, if they have money, most of them get it from their parents anyway, or some sort of trust fund, and that would logically cover the tuition and whatever school supplies they need. 

And since credits are for upgrading your characters weapons/abilities, why would we need an item shop that uses lien? I don't really see the appeal of it, I'd probably never use it. All the items in the item shop seem to be things you could either have just bought with credits or acquired them IC without the use of a system at all. 

Honestly it feels like it will make things more complicated and a hassle when it doesn't need to be.