Some things vary. I've seen grade schools start up at end of August. Here, it's the second day after US Labor Day (first Monday in September), and gets out the Thursday following the 21st of June.

The schools I went to, were divided into grades 3 different ways:

K-6 / 7-9 / 10-12
K-5 / 6-8 / 9-12
K-4 / 5-8 / 9-12

I wouldn't suppose there's not any place that divides K-5 6-9 10-12. XD or something weird like that.

The line between "college" and "university" is rather blurry here, too...

Moll.