Gumball Machine

Roughly 15 years ago, I found a vintage Carousel gumball machine in the trash across the street.  I absconded with it.  It was in great condition, but could be operated with any US (and probably foreign) coin; pennies, nickels, dimes, and quarters all allowed the lever to be drawn to the left, releasing whatever goodies lied therein. Being the wannabe businessman that I am, this frustrated me.

Only now have I addressed this issue.  After about an hour of tinkering with and machining the moving part which interfaces with the coins, dimes and pennies would drop straight through the machine, and wouldn't allow the lever to engage.  Another thirty minutes of Dremel-ing, and nickels passed through as well.  Now only quarters can work the machine.  

Success.

This baby will yield high profits, not to mention rob cheapskates of their lesser coins.

Of course, the true purpose of this project is so I can exploit my students.

...

Well, not really.  I know I can't get away with selling gumballs in class (chewing gum is against school rules) but there must be some way to use this as an incentive that benefits teacher and student in my middle school classroom.  

Naturally, the profits would go toward something noble.  As a class we might decide to donate the money to a charity at the end of the year, or use it to buy something neat for the classroom.

But, what do I put in it?

 I considered there might be a way to fill it with vouchers of some sort, so if a student needs a pencil, he must first pay for a voucher at the machine and then bring it to me to receive his pencil.  That idea is certainly pointless (I could just ask for a quarter) but seems fun, especially if I make going to the gumball machine a publicly humiliating ritual. And, charging for pencils is something I can do.  

The vouchers could be used for other things too.  Students might need to earn the ability to pay for a voucher, then collect them and use them at opportune moments to receive prizes/privileges in class.

I'm sure I'll figure something out.  

And heck, my students are silly enough to put money into it when it's empty, just to see if it works.

Any badass suggestions?