Another thing to consider is planning and resource time for the teacher. As an educator, I wouldn't mind if 8-5 included time for planning. Then I would have time to attend grad school at night. But if I had to teach 8-5, and then grade papers, talk to parents, and deal with administrative issues, after that, where's the time for professional development, let alone family time?
Add to that, when does an educator take the time to improve themselves and further their career?
8-5 and only two weeks off in the summer? A teacher can't attend a grad school class forget a grad school program. 
Maybe SOME teachers couldn't with that schedule, but I know many who do. Most of the faculty at my daughter's former school were in grad school, and they worked longer than 8-5.
Now that I think of it, most working adults work 8-5 or longer, and many are in grad school. We may not have to grade papers and everything else teachers do outside of 8-3, but many of us take work home, work PT jobs, have children, spouses, etc. and can still manage grad school.
...not trying to be the expert; just presenting another opinion.
Oh and btw, the 8-5 programs aren't like the 8-3 schools... this is also in response to something Koda asked but I don't feel like finding it.
The 8-5 day includes what would otherwise be considered extracurricular, study hall, counseling, etc. I doubt it works the same in every school, but in my daughter's school, her teachers had at least an hour (could've been more) to do the paper grading and other administrative stuff.
Another thing about her school is that parents were required to commit to a certain number of volunteer hours per quarter. It was a small number, like 3 hours, I think, but it helped. We graded papers, monitored the class during tests, kept the kids quiet, monitored recess, etc.