I have a niece in 4th grade and a nephew in 8th, both at ps. Both of their schools have a "good behavior party" every quarter and a "good grades party" every quarter. Additionally my nephew's middle school has a quarterly "zero tardy party" for kids who were on time to all of their classes.
I don't support this type of incentive system at all. The kids are learning to get good grades NOT because it will go far in life, open many more opportunities up to them, and help them go to a great college if they choose that path, but because they'll get a party for it. This is completely the wrong message to be sending, IMO, and I don't understand why the teachers even promote these types of extrinsic incentives, let alone expect them to really work. I sincerely doubt that a student who is exhibiting unacceptable brhavior, or who has absymal grades, or who is consistently tardy, is suddenly going to become a "perfect student" merely because they'll get a 45-minute party.