Well. I put MONEY for different things together myself, BUT that's in businesses where EVERYTHING have receipts, so I just need to keep the BOOKKEEPING separate, so I know for sure how big part of the money belong to which.
But in your case big part isn't receipts, I suppouse, which complicate things.
Your case need to collect proves and arguments talking for your side, such as:
/SHOW how he got HIS money.
/Is the canteen selling anything, which aren't sold, because it was damaged? Stolen?
/Demand to see how THEY have counted, and check if you find any errors in it. To become fair they need to let you see their counting, and you need to let them see yours, so both sides can look for errors.
Is it a "neutral" school board or something which both sides find fair to let decide the case?
Otherwice the baranggay has a "mini court", which I believe can judge in such cases too.