The3dm, I'm sure that your school is having to "lease" a field and services from OSP and I'd bet that expense was not budgeted. So, the school will have to try to make up for that unforcasted expense.
If you charge a gate fee and that money goes to the Athletic Department who in turn uses it to pay off the expense to OSP, what would it matter? I would do it this way. Some schools utilize athletic passes that are good for any school home athletic event during the school year. If OSP is collecting the fee, then these passes would not be valid, I would think. Just a thought..