When my oldest son started JK, I lived in the county. He went a full day, 3 days a week one week, 2 days the next week. Fridays were alternated. Like Mini mentioned, it was this way because the kids had to be bussed in.
When my youngest son started JK, he went every day for a half day in the morning. But when he started we lived in the city, and only a small amount of kids were bussed in. Majority were driven in by their parents or walked to school.