We sort of do have examples of this having history.happened in
For example, prior to World War II, one-room school houses for everyone aged 5 to 18 were still common. Outside of that, the normal setup was elementary school lasting up until 8th grade, followed by high school.
Middle schools, originally known as junior high schools, didn't start to become a thing until the 1960's.
In the 1970's, far fewer people than now thought less of you for not having a high school diploma, because dropping out was so common back then.
So as we progressed from a two school model to a three school model, the dropout rates decreased. Coincidence? I think not.