I think your thread is getting derailed.
The way to make the change is to be honest and straightforward about your reasons for doing so and just do it.
If there is a committee running the fund raiser, discuss it there. Otherwise, do it at your regular meeting.
Anonounce that you are planning to make some changes to the big fundraiser and encourage people to attend. If an overwhelming majority wants things the same, maybe you should reconsider.
Some people will likely be opposed to the change. They will think that rewarding participation is important.
Just as many or more will likely agree with you.
Your first test as a leader.
As an aside, I don't see a problem with a party/incentive for those who meet a minimum level of participation, especially if that is set really low. A whole school goal is a fine idea, too. It just depends what you think will work at your school.
We do a classroom incentive (50% participation gets the class in) and a minimum sales incentive (a few items) as well as the company's individual incentives.