Why Two Years of Algebra? Why ten months?

Shawn Cornally asks in this post, "Who decided that it takes an equal amount of time to teach every subject?"

No one decided.

What happened was that the time involved was "decided" upon by each school on its own and each one gradually came to the same conclusions: The summer months were too damned hot in NYC and all points south of Boston - since that was where the decision-makers were, that was the decision. Time off for major holidays and the occasional week or two here and there - Thanksgiving, Christmas and Easter - and the rest is school.

Sorry to all you "It's all based on the agrarian calender" believers - it's obvious you were never on a farm. The summer months are busy (the definition of farmer is 'a busy man') but nowhere near as busy as other times of the year.  Planting and harvest are kinda important, and that's May and October. The winter months have their own flow - it's not like farmers are sedentary during all but the 10 weeks of summer break!

The class time has varied from 40 minutes to 1.5 hours, yet everyone who thinks about it usually comes to the same conclusion: "If I have to choose, it's 50 minutes, all year. That seems the best compromise for all the students." Since the classes are all mixed up, you have to find a compromise.

Once the times were decided, the courses expanded or split to fit the available hours. If a school tried a new initiative, the courses all shifted to fit. Books were written to fit the prevailing choice with a couple extra chapters of overlap so they could sell them to more people. Teachers who finished something early would add a topic or expand a earlier one. Teachers who went over made some resolution to move faster. Look at the way AP is 1 month shorter than the rest of the school yet somehow those teachers make 2x the work fit.

Then the time became flexible and the courses changed again, slightly. A chapter was added to the math class, another Shakespeare play was included.  It was decided that 1 year was too little for all the Great Ideas of Algebra, so let's do 2, etc.

When block scheduling became all the rage, the total hours were kept constant and the material shifted to fit the new format. If the course syllabus changed radically, the new would always be compared to the old because the old needed to be fundamentally similarity to the new.

Shift, change and adapt --- Evolution, baby.

Additionally, the expectations of a particular course become centered around the transcript, i.e, does a grade of "A" in a course labeled "CP Physics" taught by a guy named Cornally in the Midwest using SBG mean what I think it means and include what I think it should include?  Do I schedule this kid for a remedial physics class before allowing him to matriculate or do I throw him right in and let him become a Physics major at my Uni?  Do I hire him based on this report or not?

Given all that, it's surprisingly consistent from student to student. Match up the "A" students in CP Physics from 80 different schools and they'll be amazingly similar and their abilities and knowledge pretty damned predictable.