What are treasure parcels? (is that a 4ed thing?)
So what do I tell my players? "Yes you spent several tens of thousands of gold setting up the school and its being successful but I'm not going to give you anything extra for your efforts. You can go solve this adventure based around your school and when you finish it you'll get the same treasure that you would have gotten if you had just done the adventure anyway?"
Seems kinda lame and destroys the suspension of disbelief in my book.
Like I said these are brand new players so its not like they're doing the iron pot D&D trick or something they just wanted to set up a cool school and expect it to turn some sort of profit.
Treasure parcels are in the DMG. If you want, you could boost the amount of treasure per parcel.
The thing is you can't make up a detailed rules set to simulate running a fantasy business that follows all of the following criteria:
Created in a short amount of time
Reasonably complex to make it interesting to play
Balanced
These things take time and I don't know of ANY rpg that really has good rules for running a business. You should try to frame it within the rules of the system you're playing in. Skill challenges, milestones, etc.
However you make a good point about the investment of gold -to give the players a bonus for investing their money you might give them several benefits
For example an investor PC could get all of the following
A +2 bonus to all social skills made against people of the city - they are important people now
Free mundane equipment and services - bribes from parents and students, school assets the PCs can use etc
A utility encounter or daily power based on their role in the school
A teacher could get
Do as I say Not as I Do
"No, no no. You're doing it wrong! Correct for windage!"
Daily - Ranged 10 - immediate interrupt
The character points out an error in an ally's attack and tells them how to correct the error
effect: One ally within range may reroll a failed attack roll. The ally must keep the second roll.
A manager/superintendent/investor could get
"We run things around here"
Plutocrat's Confidence
Aura 5
All allied characters within range gain a +1 bonus to their Will defense.