|I think it's worth taking up with the school/school board - whoever entered into the contract. Even if they need the corporate funding, those companies have lots of beverage options. The schools don't have to allow soda. And yes, I know the sugar content is just as bad in juice and bottled water has lots of other issues. Either is a better option than soda though, which has no redeeming value at all.
I think in the short term you contact the organizers; the school board afterwards. It breaks down into this year's
festival and future festivals/events.
I believe an important point to make, should you choose to emphasize health rather than the corporate funding angle, is that there are healthier options even from the Pepsi company.
Another set of questions I'd immediately have is this: who is organizing the Fall Festival? Teachers or PTA? Then ask the person in charge what the activities and funding goals are. In terms of activities, is it kid-oriented or adult oriented? If you have philosophical objections to soda, how else could you contribute. That's a question for whoever is heading up the organization of it. And I'm betting it's not the teachers--they have way too much to do--but perhaps a PTA committee.
In my recent experience with having a really strong opposite opinion, the approach that got good response was one that combined politely worded objections with constructive suggestions.
If letters are already going home, then it's too late to change the whole thing for this year. Can't make it soda-free. So you have to decide whether you do want to contribute to the event and if so, what you think a reasonable compromise would be. For example, you may not have a kid in 5th grade, but you're bringing water anyway. Or you won't contribute soda, but you'll give time (more valuable) or some other thing.
Highly doubt you're the only parent with objections.
A standard response to someone with complaints is to invite them to join the committee. It's occasionally (often?) a cynical response--not believing that the person will actually follow-through. Still, would be good to think about what level of involvement you want in school events.