Compared with the 1950's and 1960's, students have more material and subjects to cover. As each year passes, there is more history to learn.
Even mathematics changed, and more than simple addition and subtraction. For income tax for example. Compare the income tax forms from the 1960's with the forms for 2018.
Instead of increasing the years needed for high school education, Mike Harris dropped grade 13. Which meant there is less time for the new subjects and material we need these days.
The sciences in the 1950's didn't include the astronomy we now know, and continuing to learn today.
Everyone should be learning "keyboarding" or "computer skills", with our handheld computers (smartphones or tablets), which we didn't need to learn in the 1950's or 1960's. I remember when typing was not taught for academic or technical students, only for business students.
I attended high school in the 60s and we definitely did take astronomy. Of course, like all sciences and technology subjects, there have been significant advances. As well, while the tax forms certainly have become more complex, the math has not; it is still basic addition and subtraction with a bit of percentage work thrown in. It's not the annual reporting that's the problem but the financial literacy that surrounds it that is lacking.
A big problem impacting contemporary public education compared to days of yore is the expectation that it fill social gaps. Other than perhaps health and 'home ec', in the past the system didn't have to cover matters such as morality, equity, diversity, social justice, etc. I'm not saying those are unimportant topics, but, back in the day', they were not seen as part of the public education system's mandate.
Expecting teenagers to get excited about topics such as history, literature and grammar will always be a tough sell, but properly constructed and communicated ideas are just as important now as before. If anyone thinks an unstructured stream of consciousness, unencumbered by things like punctuation and sentence structure will win than job, sales contract or grant application is in for a surprise.
I did appreciate the comment that the socialization of education has led to the diminishment of the capable rather than the uplifting of the more challenged. The goal should be to equalize (or at least maximize the potential of) opportunity, not outcomes. The will always be future PhD candidates and folks that repair plumbing in every school, but if each was given the chance to explore and maximize their individual aptitude and goals, then the system has largely worked.
Part of the problem is the system, like so many others, has been taken over by academic professionals rather than, as it used to be, by people who started as teachers and worked their way through to administration and into the bureaucracy.
As for keyboarding skills, from what I've seen, thumb typing and exploitation of platforms is doing just fine without formal training. Understanding how any of it actually works, or using it responsibly, is possibly another thing.
All of this has virtually nothing to do with the original topic of classroom duration. Consideration of later calendar starting or stopping would have to take into account shifting climate patterns. Perhaps a shorter break at Christmas to help achieve the desired number of days. Of course there is always the matter of 'professional development' days.