We don't celebrate or reward those. DD is expected to bring home a green slip each day just as I am expected to obey the speed limit when driving each day. Green slips are (generally) neutrally commented on.
I personally think that's exactly right. My DS5 has never had a yellow (caution), blue (major infraction, but one-time), or red (repeated offenses in a day) card at school (they don't bring them home), and his response when I ask him if he's ever had one is always: "No!" with an inflection of 'well of COURSE not!'. We will be the same way with our DD4, who, ahem, does not ALWAYS make perfect choices... (although usually).
