Are these differences from state to state only, or schools do whatever they want?
Even schools in the same district are drastically different from one another.
America has a pretty lax attitude toward education, from what I've seen. Maybe in other places it's different, but every school I've ever been to was a joke. The "teachers" were hired straight out of college, sometimes they're hired while still in college (i.e. my 11th grade chemistry teacher), they're paid virtually nothing so very few of them stayed for more than a year, the ones that did stay stayed because they're so psychotic they couldn't get work elsewhere and they spent the majority of class lamenting the passed opportunities they'd missed (or concocting off-the-wall escape plans, i.e. my 11th grade precalc teacher who'd planned to triple his life savings at the blackjack tables in Reno and "blow this hell-hole", or with such an inferiority complex that they never attempted to do anything, i.e. my 11-12th grade German teacher, who was actually a business major, but let's not get into that). Needless to say, none of them wanted to be there and they made to attempt to put on a happy face.
You talk about an unequal grading scale between the states, there was a totally different scale in every class, and some teachers couldn't be confined by a standard 0-100 numerical scale, they had to make up their own scale based on a combination of numbers, letters, and obscure symbols that only they could decipher. It made compiling a report cards a near impossibility. I liked my first German teacher (he was transferred to French after the French and Spanish teachers quit, the new Spanish teacher came from the junior high school, which was left sans a French/German/Spanish program since she taught them all), he just came right out and said that the grading system was a farce and that his grades were derived from random guessing an favoritism. He had a mental problem of some sort (he had a very vague grasp of time and he'd often go off on flights of fancy, inventing whole worlds in his mind and drawing scenes from them on whatever paper he could find, then he'd disappear for a couple days and return as though nothing had happened. He also carried an old, battered "Ellis Island"-type briefcase wherever he went. In it was only a pink Barbie lunchbox containing a banana), but he was an overall likable guy when he wasn't provoked.
Our first principal, who'd been principal of the school since it opened, I think, actually went insane two years into my being there. He walked around with a walk-talkie (the school was one three-story building, with a single hallway on each level, comprised of maybe fifty classrooms, you can yell at one corner and be heard from the opposing) with no batteries in it, and he used it to talk to people across the room from him. He had been steadily running the school into the ground, squandering the $70 or so million dollar budget in a few days, then leaving the school to run off of $600 for the rest of the year. He was finally replaced with a pedophile that was no better with money than he was, who, as far as I know, is still there.
It all sounds so funny in hindsight, though it really wasn't living it. What was the question again? Study time? None, studying didn't pay. Three-fourths the time the teacher'd forget to make the test, or he might just not of felt like it (our first German teacher would declare a Chess-day when he didn't feel like working, when he and everyone else would play chess for the period, it was his favorite game) or the test would be so random that studying just wasn't possible. Homework was rarely collected after it was assigned so it didn't pay doing it, either. On the rare times it was asked for, it was usually simple enough to do during class. After all, it didn't have to be right, you had a guarantee that it would never be read. So, um... yeah, none.
Someone mentioned AP classes in another thread, saying that they should be more standardized. We didn't have many, AP English, AP Calculus... that was about it. I don't know about AP Calc, I just took regular calc, but AP English was no different from any other class.
That only scrapes the surface, I haven't even touched on junior high or elementary school. Oh, I've had a very sad education. I should write a book, I'm sure it would be made into a made-for-TV movie or possibly a miniseries.