First off, there's elementary school. Children start off in Kindergarten, then first grade, and so on up to fifth grade.
After elementary school, children spend two years in "middle school".
Then they spend four years in high school. (I believe the four years are called freshman, sophomore, junior and senior.)
I don't have any experience as yet about middle and high schools, and I've only got experience of one elementary school. But so far I'm really impressed. First of all, it's big. But it's good big: it's really well spaced out. They've got a huge playground, lots of big open playing fields, and loads of slides and climbing frames and whatnot.
Although kindergarten is roughly equivalent to "P1" in Northern Ireland, there's about six months difference in the cutoff dates. So children in Kindergarten are typically six months older than they would be in P1. Also, sometimes parents here will hold their children back a year just in case they would be too young. (Which is pretty much unheard of in NI - at least, I haven't heard of it.)
The level of education? The two schools our oldest child has been to have both struck me as great. I've been really impressed with the teachers in both places.
As with everything I write about, I could be totally wrong. Comments welcome.