Why does school measure hours instead of skills?

The short answer

Around 1906 a foundation defined the standard unit of education as time, not skill, and the money has followed the hours ever since. Once time is the unit, a school paid by the hour is built to produce hours.

The reasoning

The Carnegie Unit, set around 1906, measures hours of instruction. A century later it's still the basis of the credit hour. It's a receipt for time. It was never a measure of what a student learned.

Follow the money from there. Think about a mechanic who's paid by the hour. They have a quiet reason to take longer. A school paid by the hour has the same reason to keep you there. A student loan pays the school in full whether or not the student learns anything or is ever hired. Nobody planned this. Money tends to get what it rewards.

The shape of the day comes from the same history. The bells, rows and age grades came from the Prussian model, which Horace Mann brought to Massachusetts. Rockefeller's General Education Board put well over a hundred million dollars into American schooling. Standard units and standard schedules made a big system possible to run.

The effects show up in the data. Full-time students studied about 24 hours a week in 1961 and about 14 in 2003. A demanding course costs money: someone has to read the papers and defend the C. An easier one costs less.

This is a story about incentives, not villains. Most people inside the system are good people paid to do the wrong thing. Teachers don't set the credit hour. Nobody in the building has to want the outcome.

What would change it? Pay for what a person can do, not for hours in a seat. Medicine is already making that move.

The evidence and sources

The Carnegie Unit, set around 1906, measures hours of instruction and is still the basis of the credit hour. Elena Silva, Taylor White, and Thomas Toch, The Carnegie Unit (Carnegie Foundation for the Advancement of Teaching, 2015).
The bells, rows, and age grades came from the Prussian model, brought to Massachusetts by Horace Mann. Horace Mann, Seventh Annual Report (1843).
Rockefeller's General Education Board put well over a hundred million dollars into American schooling. General Education Board, An Account of Its Activities, 1902-1914 (1915).
A student loan pays the school in full whether or not the student learns or is ever hired. William Bennett, "Our Greedy Colleges," New York Times, February 18, 1987.
Full-time students studied about 24 hours a week in 1961 and about 14 in 2003. Philip Babcock and Mindy Marks, Review of Economics and Statistics 93, no. 2 (2011).

What to do with this

When you look at a program, ask what it measures. Hours in a seat, or something a person can show? Prefer the one that ends with a skill you can demonstrate.

Where this falls short

Standard units did make large systems workable, and some of the people who built them were solving real problems of their day. The point is the incentive that's left behind, not the intent of anyone who built it.

Also asked as

What is the Carnegie Unit?
Where did the modern school system come from?
Why hasn't school changed in a hundred years?

The ideas behind this answer

Paid for time, not skill
School is funded and measured by hours in a seat, so hours are what it is built to produce.
The repeat customer
A college that sells you a degree that doesn't get you hired often gains you as a customer for the next degree, and an easier course costs less to run.
Paid by the gate
The people who decide which credentials count are paid by those credentials.