Can someone explain the American Highschool system for me?

I'm writing a book that takes place somewhere in America and they have things like homerooms, scheduals, grades, sophmores, senious. It's confusing, can anybody explain it to me? I live in the UK if you havn't guessed lol. Like, if you could write something like "homeroom = clroom" that would be reeeeally helpful. I'm not to sure if this is in the right catagory.