J.A.
Yes, kinder is generally a 3 hour program (some schools have full day kinder, mostly private). Once students get into first grade it is a full day. Kinder is not mandated/requried by the state. Parents do not have to send their children to kinder and can start them in first grade, although I do not know of anyone who would do that, especially with what is required for 1st graders to be able to do. Kids need the foundation of Kinder to help build their future skills.
I taught 1st grade for 7 years and I have twins who will be starting Kinder next year. Personally I think they would do great in a full day program, although it is a long day for many kids that age. When I taught 1st grade, the first 3 weeks were really hard. Many of the students had a rough time with the full day.
J.