IN the US, preschool is generally privately paid by parents, and can be any or all of the years ages 2-5. (This is different than daycare, which is childcare while parents work.) Preschool is most commonly half day, though sometimes all day (especially as children approach "real school" age), anywhere from 2-5 mornings or afternoons per week. It can be a community-based coop, a church outreach, or quite expensive private school. It's the whole gamut. It's basically socialization, learning colors/numbers/prereading skills.
Because it's privately paid and seen as a worthwhile pre-educational experience, the gov't. authored "Head Start" programs starting in the
1960s to fill the gap between children of parents who could and parents who couldn't afford preschool. It was felt that students who had pre-school were more "ready" for learning in first grade.
When I started school in 1960 (I'm showing my age!), kindergarten was typically the first school experience. Few children went to preschool in the little working-class town where I grew up. DH, who grew up in a city and was the child of academics, did go to preschool - a parent-driven coop preschool, IIRC.
In my experience, schooling in the US was grades 1-12 (ages 6 through
17/18). A lot of school systems offered kindergarten as a pre-school socialization - learning letters, how to line up, ordering the day (pledge of allegiance, weather/calendar, etc.). Kindergarten attendance was not legally REQUIRED until as recently as the 1970s/80s, depending on where you lived. (Education requirements were state-mandated, not federally so.)
In the 1990s, there began a push for optional pre-kindergarten and mandatory all-day kindergarten, and these were, I believe, part of the "No Child Left Behind" education reform of the GW Bush years. All-day kindergarten and half-day pre-kindergarten is now the norm in the public schools, to my knowledge. I don't think pre-K is mandatory; it's an option for parents who use the public schools rather than private preschool. Many of the preschools offer kindergarten, and parents often (in my area, anyway, it could be different elsewhere) keep their children in the private preschool from age 2 or 3 through kindergarten, then start the public schools with 1st grade.
Each state used to have its own deadline for age before school entrance. Here in MD, it used to be 12/31 of the year the child entered school. Therefore a child could start kindergarten at age 4-3/4 and first grade at 5-3/4. For some children, this seemed too young for the pressures of all-day school. Some private schools began to offer "pre-first" grade, a developmentally appropriate growth year between kindergarten and first grade. I elected for this with my son, who had a late October BD. (In contrast, where I grew up in NJ, the child had to be five by 9/30 of the year he/she started kindergarten. And in the past decade or so, Maryland has rolled back the age cutoff...I believe it's now sometime before my young niece's BD in late October, because her parents were perturbed she wouldn't be able to start public school "on schedule" and would therefore be one of the oldest students in her grade.)
School attendance is mandatory through age 16, I believe, meaning at age
16 a student can legally "drop out" if they don't intend to graduate. There's a move afoot to change this to age 18, but I don't think it's yet in force.
In most places, public school is: Elementary: Pre-K through grade 5 Middle school - grades 6,7,8 High school, grades 9-12
There is a move now in many places to revert to K-8 elementary schools, and do away with middle schools.
Hope that helps! And everyone chime in if your area's system is substantially different.
sue