Credits are sort of like digital currency; I imagined them as being contained in a sort of "digital profile" for students that was accessible from anywhere within the school or from a device like a phone/watch/etc. If the student plans to exchange them for money, there are ATM style things that would let them select their currency value and then transfer the credits, which would then print out as actual money (not freshly made money, as printing bills whenever one needed it would destroy the economy) for them to use.

Everyone who takes a mission gets those credits as a reward. Loot from battles, however, has to be divvied up. 

I'll work on a simplified map and start adding some stuff to the google doc to answer other questions; sorry, I should have had the setting more fleshed out. Are there any other questions that I should probably add info about?